Jayaraman, Bhaskar
2008-Aug-18 19:27 UTC
[Xen-devel] Passing a grant reference to another domain!
Hi can anyone point me to the code which passes grant reference between domains? In Mini-OS it doesn''t seem to be used and I''m a little clueless where to look for passing of reference. I am aware of the way a grant table is mapped, how to use the ring buffer once you map the shared memory page into your domain using the reference and the domain id, and how to read and write into the xenstored database. However, I''m struggling to understand how we pass the grant reference to a particular domain once you share a page after setting necessary access permissions on the page. Regards, Bhaskar. _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Daniel Stodden
2008-Aug-18 20:41 UTC
Re: [Xen-devel] Passing a grant reference to another domain!
On Tue, 2008-08-19 at 03:27 +0800, Jayaraman, Bhaskar wrote:> Hi can anyone point me to the code which passes grant reference between domains?> In Mini-OS it doesn''t seem to be used and I''m a little clueless where to look for passing of reference. > I am aware of the way a grant table is mapped, how to use the ring buffer once you map the shared memory page> into your domain using the reference and the domain id, and how to read and write into the xenstored database.> However, I''m struggling to understand how we pass the grant reference to a particular domain once> you share a page after setting necessary access permissions on the page.XenStore''s typically used for bootstrapping shared memory. For generic I/O rings, it''s sufficient to set up sharing of the ring itself. In that case, a frontend allocates the shared page and puts that reference into an xs location watched by the backend. Beyond that, further references would be typically be sent as messages, but when and how many clearly depends on the application and protocol. See e.g. the sector segment descriptors in xen/include/public/io/blkif.h. Similarly for io/netif.h, but blkif is simpler. HTH, Daniel -- Daniel Stodden LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation Institut für Informatik der TU München D-85748 Garching http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jayaraman, Bhaskar
2008-Aug-19 07:54 UTC
RE: [Xen-devel] Passing a grant reference to another domain!
Daniel thanks for the reply, I have a few more q''s. 1] A grant reference number is the array index in the grant table? 2] Passing more than one reference would require more than one key in the XS and the sharing domain will simply populate values and keys in xs, that are assumed to be known to the domain with which it is sharing those references? 3] The reason I''m asking you this is because I want to do so in an HVM which is a requirement for my project. I am not sure if libxenstore is available for an HVM?? Also since an HVM cannot use the backend mechanism that blkfront and netfront drivers use, I will have to transfer pages using grant references on my own between domains if I''m right?? This is for making I/Os. 4] Plus if I want to transfer many pages, I will have to keep increasing my memory allocation with Xen by the number of pages that I lose as a result of sharing those pages and the shared domain will have to free up the shared pages once used to remain within its prescribed memory allocation range? 5] To pass a grant ref I guess I''ll be doing the following: - In the sharing domain: - xsh = xs_domain_open() xth = xs_transaction_start(xsh) xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref", "2045", 5); xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref", "2046", 5); In the shared domain: - xsh = xs_domain_open() xth = xs_transaction_start(xsh) char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref", NULL); char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref", NULL); Would this be a way of passing references between domains? 6] If so, the above code uses libxenstore and this is is user space. Is there a way to do this from kernel space? Regards, Bhaskar. -----Original Message----- From: Daniel Stodden [mailto:stodden@cs.tum.edu] Sent: Tuesday, August 19, 2008 2:12 AM To: Jayaraman, Bhaskar Cc: Xen Developers Subject: Re: [Xen-devel] Passing a grant reference to another domain! On Tue, 2008-08-19 at 03:27 +0800, Jayaraman, Bhaskar wrote:> Hi can anyone point me to the code which passes grant reference between domains?> In Mini-OS it doesn''t seem to be used and I''m a little clueless where to look for passing of reference. > I am aware of the way a grant table is mapped, how to use the ring > buffer once you map the shared memory page> into your domain using the reference and the domain id, and how to read and write into the xenstored database.> However, I''m struggling to understand how we pass the grant reference > to a particular domain once> you share a page after setting necessary access permissions on the page.XenStore''s typically used for bootstrapping shared memory. For generic I/O rings, it''s sufficient to set up sharing of the ring itself. In that case, a frontend allocates the shared page and puts that reference into an xs location watched by the backend. Beyond that, further references would be typically be sent as messages, but when and how many clearly depends on the application and protocol. See e.g. the sector segment descriptors in xen/include/public/io/blkif.h. Similarly for io/netif.h, but blkif is simpler. HTH, Daniel -- Daniel Stodden LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation Institut für Informatik der TU München D-85748 Garching http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Daniel Stodden
2008-Aug-19 18:21 UTC
RE: [Xen-devel] Passing a grant reference to another domain!
On Tue, 2008-08-19 at 15:54 +0800, Jayaraman, Bhaskar wrote:> Daniel thanks for the reply, I have a few more q''s. > 1] A grant reference number is the array index in the grant table?Yup.> 2] Passing more than one reference would require more than one key in the XS> and the sharing domain will simply populate values and keys in xs, that> are assumed to be known to the domain with which it is sharing those references?Yes, the key paths need to be known by convention. Note that access privileges need consideration. This is trivial between dom0 (owning the whole xenstore) and and domU (owning a respective subtree). Not sure about different pairings.> 3] The reason I''m asking you this is because I want to do so in an HVM which is a requirement> for my project. I am not sure if libxenstore is available for an HVM?? Also since an> HVM cannot use the backend mechanism that blkfront and netfront drivers use, I will> have to transfer pages using grant references on my own between domains if I''m right??Are you really sure it really needs to be full virtualization exclusively? XenStore is available to HVM as well. I''m not sure what''s presently available for userland access. Which OS?> This is for making I/Os. > > 4] Plus if I want to transfer many pages, I will have to keep increasing my memory> allocation with Xen by the number of pages that I lose as a result of sharing> those pages and the shared domain will have to free up the shared pages> once used to remain within its prescribed memory allocation range?It quite simple: You cannot allocate more memory than you have. Grant tabs manage sharing, not allocation. Xen requires you to own the page you''re going to share. Thinking client/server, services typically employ a scheme where the client is required allocates the memory needed to fulfill his demands. Prevents DoS patterns and similar issues.> 5] To pass a grant ref I guess I''ll be doing the following: - > > In the sharing domain: - > xsh = xs_domain_open() > xth = xs_transaction_start(xsh)Transactions are only necessary if the updates to xenstore are required to be atomic. If you can do without them, don''t use them. It''s been a while since I used libxs, but if you do transactions, I believe should be an accompanying function call to commit it.> xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref", "2045", 5); > xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref", "2046", 5); > In the shared domain: - > xsh = xs_domain_open() > xth = xs_transaction_start(xsh) > char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref", NULL); > char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref", NULL); > > Would this be a way of passing references between domains?In part yes. Can the reading domain rely on the data being readily available when it''s going through that path? If not, there''s a concept called watches. Basically a callback mechanism in which you can register. A change to the key you subscribed to will let your (re-)read the new information without resorting to needless polling.> 6] If so, the above code uses libxenstore and this is is user space. Is there a way to do this from kernel space?For a PV kernel in support of libxc, all those libxc features just call into to a chardev interface backed by the kernel. But generally, it may depend on the OS you''re targeting. Daniel -- Daniel Stodden LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation Institut für Informatik der TU München D-85748 Garching http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jayaraman, Bhaskar
2008-Aug-25 10:06 UTC
RE: [Xen-devel] Passing a grant reference to another domain!
Daniel sorry for the late reply, from the mail threads in the past week I can
see that grant references aren''t supported in HVMs currently but we
require it on HVMs so we can run unmodified OSes and code like a stubdom, which
makes me conclude that stubdoms are PVs. Right?
I''m using CentOS and I''m not sure how to use libxc
because there aren''t any man pages or docs that I''m able to
look into but thanks for the tip on how the kernel is accessed from libxc.
For accessing libxenstore from the kernel I guess I''m gonna
have to go through the code and figure out which char devices these are and how
the entry points are called and maybe invoke the char s/w table func pointer
directly (still new to Linux kernel).
Thanks again.
Bhaskar.
-----Original Message-----
From: Daniel Stodden [mailto:stodden@cs.tum.edu]
Sent: Tuesday, August 19, 2008 11:52 PM
To: Jayaraman, Bhaskar
Cc: Xen Developers
Subject: RE: [Xen-devel] Passing a grant reference to another domain!
On Tue, 2008-08-19 at 15:54 +0800, Jayaraman, Bhaskar
wrote:> Daniel thanks for the reply, I have a few more q''s.
> 1] A grant reference number is the array index in the grant table?
Yup.
> 2] Passing more than one reference would require more than one key in the
XS
> and the sharing domain will simply populate values and keys in xs, that
> are assumed to be known to the domain with which it is sharing those
references?
Yes, the key paths need to be known by convention. Note that access
privileges need consideration. This is trivial between dom0 (owning the
whole xenstore) and and domU (owning a respective subtree). Not sure
about different pairings.
> 3] The reason I''m asking you this is because I want to do so in an
HVM which is a requirement
> for my project. I am not sure if libxenstore is available for an HVM?? Also
since an
> HVM cannot use the backend mechanism that blkfront and netfront drivers
use, I will
> have to transfer pages using grant references on my own between domains if
I''m right??
Are you really sure it really needs to be full virtualization
exclusively? XenStore is available to HVM as well. I''m not sure
what''s
presently available for userland access. Which OS?
> This is for making I/Os.
>
> 4] Plus if I want to transfer many pages, I will have to keep increasing my
memory
> allocation with Xen by the number of pages that I lose as a result of
sharing
> those pages and the shared domain will have to free up the shared pages
> once used to remain within its prescribed memory allocation range?
It quite simple: You cannot allocate more memory than you have. Grant
tabs manage sharing, not allocation. Xen requires you to own the page
you''re going to share. Thinking client/server, services typically
employ
a scheme where the client is required allocates the memory needed to
fulfill his demands. Prevents DoS patterns and similar issues.
> 5] To pass a grant ref I guess I''ll be doing the following: -
>
> In the sharing domain: -
> xsh = xs_domain_open()
> xth = xs_transaction_start(xsh)
Transactions are only necessary if the updates to xenstore are required
to be atomic. If you can do without them, don''t use them. It''s
been a
while since I used libxs, but if you do transactions, I believe should
be an accompanying function call to commit it.
> xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref",
"2045", 5);
> xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref",
"2046", 5);
> In the shared domain: -
> xsh = xs_domain_open()
> xth = xs_transaction_start(xsh)
> char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref",
NULL);
> char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref",
NULL);
>
> Would this be a way of passing references between domains?
In part yes. Can the reading domain rely on the data being readily
available when it''s going through that path?
If not, there''s a concept called watches. Basically a callback
mechanism
in which you can register. A change to the key you subscribed to will
let your (re-)read the new information without resorting to needless
polling.
> 6] If so, the above code uses libxenstore and this is is user space. Is
there a way to do this from kernel space?
For a PV kernel in support of libxc, all those libxc features just call
into to a chardev interface backed by the kernel.
But generally, it may depend on the OS you''re targeting.
Daniel
--
Daniel Stodden
LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München D-85748 Garching
http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel
Sandesh
2008-Aug-25 10:12 UTC
RE: [Xen-devel] Passing a grant reference to another domain!
Hi jayraman,> I''m using CentOS and I''m not sure how to use libxc because there > aren''t any man pages or docs that I''m able to look into but thanks for > the tip on how the kernel is accessed from libxc.libxc is a user-level control library which performs all the hypercalls required for an operation. libxc actually makes an ioctl call to /proc/privcmd interface of the dom0 which in turn makes the call to the hypervisor function. Handled by drivers/xen/privcmd.c . If you are trying to get some info of a domain from the hypervisor, you can directly add a new function in libxc which makes a do_domctl hypercall and add a appropriate case in do_domctl function which retrieves you the required function. I don''t know what you are trying to achieve. Its just a suggestion, which you can explore on. Thanks, Sandesh Please do not print this email unless it is absolutely necessary. The information contained in this electronic message and any attachments to this message are intended for the exclusive use of the addressee(s) and may contain proprietary, confidential or privileged information. If you are not the intended recipient, you should not disseminate, distribute or copy this e-mail. Please notify the sender immediately and destroy all copies of this message and any attachments. WARNING: Computer viruses can be transmitted via email. The recipient should check this email and any attachments for the presence of viruses. The company accepts no liability for any damage caused by any virus transmitted by this email. www.wipro.com _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
pradeep singh rautela
2008-Aug-25 10:19 UTC
Re: [Xen-devel] Passing a grant reference to another domain!
On Mon, Aug 25, 2008 at 3:36 PM, Jayaraman, Bhaskar <Bhaskar.Jayaraman@lsi.com> wrote:> Daniel sorry for the late reply, from the mail threads in the past week I can see that grant references aren't supported in HVMs currently but we require it on HVMs so we can run unmodified OSes and code like a stubdom, which makes me conclude that stubdoms are PVs. Right?Yes they are PV domains. Thanks, --Pradeep> I'm using CentOS and I'm not sure how to use libxc because there aren't any man pages or docs that I'm able to look into but thanks for the tip on how the kernel is accessed from libxc. > > For accessing libxenstore from the kernel I guess I'm gonna have to go through the code and figure out which char devices these are and how the entry points are called and maybe invoke the char s/w table func pointer directly (still new to Linux kernel). > > Thanks again. > Bhaskar. > > -----Original Message----- > From: Daniel Stodden [mailto:stodden@cs.tum.edu] > Sent: Tuesday, August 19, 2008 11:52 PM > To: Jayaraman, Bhaskar > Cc: Xen Developers > Subject: RE: [Xen-devel] Passing a grant reference to another domain! > > On Tue, 2008-08-19 at 15:54 +0800, Jayaraman, Bhaskar wrote: >> Daniel thanks for the reply, I have a few more q's. >> 1] A grant reference number is the array index in the grant table? > > Yup. > >> 2] Passing more than one reference would require more than one key in the XS > >> and the sharing domain will simply populate values and keys in xs, that > >> are assumed to be known to the domain with which it is sharing those references? > > Yes, the key paths need to be known by convention. Note that access > privileges need consideration. This is trivial between dom0 (owning the > whole xenstore) and and domU (owning a respective subtree). Not sure > about different pairings. > >> 3] The reason I'm asking you this is because I want to do so in an HVM which is a requirement > >> for my project. I am not sure if libxenstore is available for an HVM?? Also since an > >> HVM cannot use the backend mechanism that blkfront and netfront drivers use, I will > >> have to transfer pages using grant references on my own between domains if I'm right?? > > Are you really sure it really needs to be full virtualization > exclusively? XenStore is available to HVM as well. I'm not sure what's > presently available for userland access. Which OS? > >> This is for making I/Os. >> >> 4] Plus if I want to transfer many pages, I will have to keep increasing my memory > >> allocation with Xen by the number of pages that I lose as a result of sharing > >> those pages and the shared domain will have to free up the shared pages > >> once used to remain within its prescribed memory allocation range? > > It quite simple: You cannot allocate more memory than you have. Grant > tabs manage sharing, not allocation. Xen requires you to own the page > you're going to share. Thinking client/server, services typically employ > a scheme where the client is required allocates the memory needed to > fulfill his demands. Prevents DoS patterns and similar issues. > >> 5] To pass a grant ref I guess I'll be doing the following: - >> >> In the sharing domain: - >> xsh = xs_domain_open() >> xth = xs_transaction_start(xsh) > > Transactions are only necessary if the updates to xenstore are required > to be atomic. If you can do without them, don't use them. It's been a > while since I used libxs, but if you do transactions, I believe should > be an accompanying function call to commit it. > >> xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref", "2045", 5); >> xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref", "2046", 5); >> In the shared domain: - >> xsh = xs_domain_open() >> xth = xs_transaction_start(xsh) >> char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref", NULL); >> char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref", NULL); >> >> Would this be a way of passing references between domains? > > In part yes. Can the reading domain rely on the data being readily > available when it's going through that path? > > If not, there's a concept called watches. Basically a callback mechanism > in which you can register. A change to the key you subscribed to will > let your (re-)read the new information without resorting to needless > polling. > >> 6] If so, the above code uses libxenstore and this is is user space. Is there a way to do this from kernel space? > > For a PV kernel in support of libxc, all those libxc features just call > into to a chardev interface backed by the kernel. > > But generally, it may depend on the OS you're targeting. > > Daniel > > -- > Daniel Stodden > LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation > Institut für Informatik der TU München D-85748 Garching > http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu > PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel >-- Pradeep Singh Rautela http://eagain.wordpress.com http://emptydomain.googlepages.com _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Mark Williamson
2008-Aug-25 16:05 UTC
Re: [Xen-devel] Passing a grant reference to another domain!
On Monday 25 August 2008 11:06:29 Jayaraman, Bhaskar wrote:> Daniel sorry for the late reply, from the mail threads in the past week I > can see that grant references aren''t supported in HVMs currently but we > require it on HVMs so we can run unmodified OSes and code like a stubdom, > which makes me conclude that stubdoms are PVs. Right? > > I''m using CentOS and I''m not sure how to use libxc because there > aren''t any man pages or docs that I''m able to look into but thanks for the > tip on how the kernel is accessed from libxc. > > For accessing libxenstore from the kernel I guess I''m gonna have to > go through the code and figure out which char devices these are and how the > entry points are called and maybe invoke the char s/w table func pointer > directly (still new to Linux kernel).You can access Xenstore functionality directly from an in-kernel API in either a PV domain or an HVM domain with PV drivers installed. Would this be adequate for your needs? You probably shouldn''t need to hijack any userspace APIs for this part of your work. The in-kernel API to Xenstore is called "Xenbus". Interesting files for you include: drivers/xen/xenbus/* include/xen/xenbus.h drivers/xen/{blkback,netback}/xenbus.c drivers/xen/{blkfront,netfront}/*.c Hope this helps, Cheers, Mark> Thanks again. > Bhaskar. > > -----Original Message----- > From: Daniel Stodden [mailto:stodden@cs.tum.edu] > Sent: Tuesday, August 19, 2008 11:52 PM > To: Jayaraman, Bhaskar > Cc: Xen Developers > Subject: RE: [Xen-devel] Passing a grant reference to another domain! > > On Tue, 2008-08-19 at 15:54 +0800, Jayaraman, Bhaskar wrote: > > Daniel thanks for the reply, I have a few more q''s. > > 1] A grant reference number is the array index in the grant table? > > Yup. > > > 2] Passing more than one reference would require more than one key in the > > XS > > > > and the sharing domain will simply populate values and keys in xs, that > > > > are assumed to be known to the domain with which it is sharing those > > references? > > Yes, the key paths need to be known by convention. Note that access > privileges need consideration. This is trivial between dom0 (owning the > whole xenstore) and and domU (owning a respective subtree). Not sure > about different pairings. > > > 3] The reason I''m asking you this is because I want to do so in an HVM > > which is a requirement > > > > for my project. I am not sure if libxenstore is available for an HVM?? > > Also since an > > > > HVM cannot use the backend mechanism that blkfront and netfront drivers > > use, I will > > > > have to transfer pages using grant references on my own between domains > > if I''m right?? > > Are you really sure it really needs to be full virtualization > exclusively? XenStore is available to HVM as well. I''m not sure what''s > presently available for userland access. Which OS? > > > This is for making I/Os. > > > > 4] Plus if I want to transfer many pages, I will have to keep increasing > > my memory > > > > allocation with Xen by the number of pages that I lose as a result of > > sharing > > > > those pages and the shared domain will have to free up the shared pages > > > > once used to remain within its prescribed memory allocation range? > > It quite simple: You cannot allocate more memory than you have. Grant > tabs manage sharing, not allocation. Xen requires you to own the page > you''re going to share. Thinking client/server, services typically employ > a scheme where the client is required allocates the memory needed to > fulfill his demands. Prevents DoS patterns and similar issues. > > > 5] To pass a grant ref I guess I''ll be doing the following: - > > > > In the sharing domain: - > > xsh = xs_domain_open() > > xth = xs_transaction_start(xsh) > > Transactions are only necessary if the updates to xenstore are required > to be atomic. If you can do without them, don''t use them. It''s been a > while since I used libxs, but if you do transactions, I believe should > be an accompanying function call to commit it. > > > xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref", "2045", 5); > > xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref", "2046", 5); > > In the shared domain: - > > xsh = xs_domain_open() > > xth = xs_transaction_start(xsh) > > char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref", NULL); > > char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref", NULL); > > > > Would this be a way of passing references between domains? > > In part yes. Can the reading domain rely on the data being readily > available when it''s going through that path? > > If not, there''s a concept called watches. Basically a callback mechanism > in which you can register. A change to the key you subscribed to will > let your (re-)read the new information without resorting to needless > polling. > > > 6] If so, the above code uses libxenstore and this is is user space. Is > > there a way to do this from kernel space? > > For a PV kernel in support of libxc, all those libxc features just call > into to a chardev interface backed by the kernel. > > But generally, it may depend on the OS you''re targeting. > > Daniel > > -- > Daniel Stodden > LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation > Institut für Informatik der TU München D-85748 Garching > http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu > PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jayaraman, Bhaskar
2008-Aug-26 12:21 UTC
RE: [Xen-devel] Passing a grant reference to another domain!
Thanks Mark, the Xenbus drivers you''re talking about are for a paravirtualized kernel. Any idea if these are being ported onto an HVM? If not grant references can be passed around in HVMs using TCP connects? Assuming I''m accesing XS from an HVM or PV guest, for guest A to read a grant reference from guest B''s xenstored dictionary path it will need to know the domain ID of B. There''s no way of assigning specific or known domainID while VM creation. So how will they each know what their respective domain IDs are to go and read each others'' grant references from a known key? Bhaskar. -----Original Message----- From: M.A. Williamson [mailto:maw48@hermes.cam.ac.uk] On Behalf Of Mark Williamson Sent: Monday, August 25, 2008 9:36 PM To: xen-devel@lists.xensource.com Cc: Jayaraman, Bhaskar; Daniel Stodden Subject: Re: [Xen-devel] Passing a grant reference to another domain! On Monday 25 August 2008 11:06:29 Jayaraman, Bhaskar wrote:> Daniel sorry for the late reply, from the mail threads in the past week I > can see that grant references aren''t supported in HVMs currently but we > require it on HVMs so we can run unmodified OSes and code like a stubdom, > which makes me conclude that stubdoms are PVs. Right? > > I''m using CentOS and I''m not sure how to use libxc because there > aren''t any man pages or docs that I''m able to look into but thanks for the > tip on how the kernel is accessed from libxc. > > For accessing libxenstore from the kernel I guess I''m gonna have to > go through the code and figure out which char devices these are and how the > entry points are called and maybe invoke the char s/w table func pointer > directly (still new to Linux kernel).You can access Xenstore functionality directly from an in-kernel API in either a PV domain or an HVM domain with PV drivers installed. Would this be adequate for your needs? You probably shouldn''t need to hijack any userspace APIs for this part of your work. The in-kernel API to Xenstore is called "Xenbus". Interesting files for you include: drivers/xen/xenbus/* include/xen/xenbus.h drivers/xen/{blkback,netback}/xenbus.c drivers/xen/{blkfront,netfront}/*.c Hope this helps, Cheers, Mark> Thanks again. > Bhaskar. > > -----Original Message----- > From: Daniel Stodden [mailto:stodden@cs.tum.edu] > Sent: Tuesday, August 19, 2008 11:52 PM > To: Jayaraman, Bhaskar > Cc: Xen Developers > Subject: RE: [Xen-devel] Passing a grant reference to another domain! > > On Tue, 2008-08-19 at 15:54 +0800, Jayaraman, Bhaskar wrote: > > Daniel thanks for the reply, I have a few more q''s. > > 1] A grant reference number is the array index in the grant table? > > Yup. > > > 2] Passing more than one reference would require more than one key in the > > XS > > > > and the sharing domain will simply populate values and keys in xs, that > > > > are assumed to be known to the domain with which it is sharing those > > references? > > Yes, the key paths need to be known by convention. Note that access > privileges need consideration. This is trivial between dom0 (owning the > whole xenstore) and and domU (owning a respective subtree). Not sure > about different pairings. > > > 3] The reason I''m asking you this is because I want to do so in an HVM > > which is a requirement > > > > for my project. I am not sure if libxenstore is available for an HVM?? > > Also since an > > > > HVM cannot use the backend mechanism that blkfront and netfront drivers > > use, I will > > > > have to transfer pages using grant references on my own between domains > > if I''m right?? > > Are you really sure it really needs to be full virtualization > exclusively? XenStore is available to HVM as well. I''m not sure what''s > presently available for userland access. Which OS? > > > This is for making I/Os. > > > > 4] Plus if I want to transfer many pages, I will have to keep increasing > > my memory > > > > allocation with Xen by the number of pages that I lose as a result of > > sharing > > > > those pages and the shared domain will have to free up the shared pages > > > > once used to remain within its prescribed memory allocation range? > > It quite simple: You cannot allocate more memory than you have. Grant > tabs manage sharing, not allocation. Xen requires you to own the page > you''re going to share. Thinking client/server, services typically employ > a scheme where the client is required allocates the memory needed to > fulfill his demands. Prevents DoS patterns and similar issues. > > > 5] To pass a grant ref I guess I''ll be doing the following: - > > > > In the sharing domain: - > > xsh = xs_domain_open() > > xth = xs_transaction_start(xsh) > > Transactions are only necessary if the updates to xenstore are required > to be atomic. If you can do without them, don''t use them. It''s been a > while since I used libxs, but if you do transactions, I believe should > be an accompanying function call to commit it. > > > xs_write(xsh, xth, "/local/domain/1/shm/tx-ring-ref", "2045", 5); > > xs_write(xsh, xth, "/local/domain/1/shm/rx-ring-ref", "2046", 5); > > In the shared domain: - > > xsh = xs_domain_open() > > xth = xs_transaction_start(xsh) > > char* val1 = xs_read(xsh, xth, "/local/domain/1/shm/tx-ring-ref", NULL); > > char* val2 = xs_read(xsh, xth, "/local/domain/1/shm/rx-ring-ref", NULL); > > > > Would this be a way of passing references between domains? > > In part yes. Can the reading domain rely on the data being readily > available when it''s going through that path? > > If not, there''s a concept called watches. Basically a callback mechanism > in which you can register. A change to the key you subscribed to will > let your (re-)read the new information without resorting to needless > polling. > > > 6] If so, the above code uses libxenstore and this is is user space. Is > > there a way to do this from kernel space? > > For a PV kernel in support of libxc, all those libxc features just call > into to a chardev interface backed by the kernel. > > But generally, it may depend on the OS you''re targeting. > > Daniel > > -- > Daniel Stodden > LRR - Lehrstuhl für Rechnertechnik und Rechnerorganisation > Institut für Informatik der TU München D-85748 Garching > http://www.lrr.in.tum.de/~stodden mailto:stodden@cs.tum.edu > PGP Fingerprint: F5A4 1575 4C56 E26A 0B33 3D80 457E 82AE B0D8 735B > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel