Xen devel - Nov 2007 - Are drivers in Dom0 virtualized in any way

Hi guys, I have this question: are the drivers (say for a hard disk)
running in Dom0 virtualized in any way?  For instance if a driver
wants to set up a DMA transfer, does it have to make a call to the
hypervisor in order to translate the guest-physical address it wants
to use as the destination of the DMA into the host-physical address
that the hypervisor has associated with that guest-physical address?
I''m not asking about the xen back drivers in Dom0, but "real"
drivers
that drive the hardware.

Or, does this translation happen lower down, in the Dom0 operating
system itself, which is of course virtualized?  Like maybe in some
kind of the DMA API that Linux provides?

Thanks,
Dave

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On Thu, 2007-11-01 at 09:47 -0400, David Stone wrote:
> Hi guys, I have this question: are the drivers (say for a hard disk)
> running in Dom0 virtualized in any way?  For instance if a driver
> wants to set up a DMA transfer, does it have to make a call to the
> hypervisor in order to translate the guest-physical address it wants
> to use as the destination of the DMA into the host-physical address
> that the hypervisor has associated with that guest-physical address?
> I''m not asking about the xen back drivers in Dom0, but
"real" drivers
> that drive the hardware.
regular PV domains like dom0 see machine addresses, as opposed to a
''pseudo-physical'' (i.e. linear) virtualized machine address
space.

this is different from full virtualization. PV domains are
virtualization-aware in that respect. they don''t care if part view on
physical memory is fragmented. (they do work a lot on a linear
representation, but that''s solely the domain''s business)

note that this does not mean that dom0 is capable of just mapping these
pages. generally, the real page tables belong to the VMM. this applies
to domUs as it does to dom0, as a a basic security constraint.
all mappings are validated by xen, including those to I/O memory. the
difference is whether they succeed (for a sufficiently privileged domain
or dom0) or don''t (for an unprivileged domain).

that said, there are variants regarding who is mapping which address
space to what. but the above is the regular case which applies to a
dom0.
> Or, does this translation happen lower down, in the Dom0 operating
> system itself, which is of course virtualized?  Like maybe in some
> kind of the DMA API that Linux provides?
uhm, well, both is true, depending on what you look at.

the linux code generally applies some variable degree of translation
between a page and the the ''bus address'' delivered to the
device. this
is due to the fact that, even for native kernels, on some architectures
the host view to physical memory can be different from the ''bus
view''.

xen exploits that by re-defining some of the macros in a pv-specific
fashion. and some translations can be as complex as they look like.

if you''re studying the dom0 code, you''ll see lots of variables
named
''mfn'' and ''pfn'', and macros like
phys_to_machine() and the like.
pfn is the linear representation, equivalent properties to what a native
kernel would a ''pfn''. even dom0 needs such a linear
representation, e.g.
as an index into the pageinfo vector. but you''d also see calls to
macros
like ''pfn_to_mfn(..)'' in the preparation of hypercall
arguments.

the general rule of thumb of thumb is: for a bare dom0 on the i386, the
memory is managed in terms of pfns, to keep the core of the kernel
happy, communicated in terms of mfns, translation is rather trivial, and
there''s not much additional translation going on beyond that.

regards,
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
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That helped a lot, I understand much better now.  Just to be 100%
certain though, the net result of the details you provided are that
the physical drivers (not the back drivers) running in Dom0 have not
been modified from their source as it is in the vanilla linux kernel?

Thanks,
Dave

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On Thu, 2007-11-01 at 12:41 -0400, David Stone wrote:
> That helped a lot, I understand much better now.  Just to be 100%
> certain though, the net result of the details you provided are that
> the physical drivers (not the back drivers) running in Dom0 have not
> been modified from their source as it is in the vanilla linux kernel?
absolutely not. in the contrary case, even ripping them off the kernel
and integrating them into xen, e.g. in order to get rid for some of the
latencies involved by keeping front- and backends in separate address
spaces, would start to look like a viable alternative.

running drivers unmodified is a major design criterion, because
maintenance cost ist a fundamental part of the overall equation.

regards,
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



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> > That helped a lot, I understand much better now.  Just to be 100%
> > certain though, the net result of the details you provided are that
> > the physical drivers (not the back drivers) running in Dom0 have not
> > been modified from their source as it is in the vanilla linux kernel?
The only difference is that functions called by those drivers (or macros used 
by them) may have xen-specific implementations.  However, the API used for 
the drivers themselves has not changed and the drivers generally need no 
changes.

Conversely, the backend drivers are aware of Xen-related issues but they just 
plumb into the block / net stack at a fairly generic layer and are unaware of 
the nature of the underlying device - Linux handles that.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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hi
  In the hvm mode, dom0  is HVM or PV? does dom0 entry into the VMM by 
using "VMexit /VMentry" instruction in intel-VT ,or not?

thanks

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tgh wrote:
> hi
>   In the hvm mode, dom0  is HVM or PV? 
Dom0 is PV.
>does dom0 entry into the VMM by using "VMexit /VMentry"
instruction in
intel-VT ,or not?
> 
No,  HVM uses "VMexit /VMentry" instruction.
> 
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dom0 is required to be PV, not HVM at the moment.  Maybe one day
this''ll
change but not for the foreseeable future, I think.

As a result, dom0 enters the VMM using int 0x82, just like other PV guests.

Cheers,
Mark

On Monday 03 December 2007, tgh wrote:
> hi
>   In the hvm mode, dom0  is HVM or PV? does dom0 entry into the VMM by
> using "VMexit /VMentry" instruction in intel-VT ,or not?
>
> thanks
>
> _______________________________________________
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> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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