Xen devel - Oct 2010 - sharing memory between qemu and domu without grant tables or vmexits...

Is it possible to share a page of memory between qemu in dom0 and domu
without using grant tables and without domu access to that page causing
a vmexit?

To set up a custom kernel debug dll under windows I need to load very
early, much earlier than my PV drivers. To have any hope of usefully
increasing the speed over serial port debugging I need to be able to
transfer data faster than 1-byte-per-vmexit...

Thanks

James

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On 17/10/2010 06:27, "James Harper"
<james.harper@bendigoit.com.au> wrote:
> Is it possible to share a page of memory between qemu in dom0 and domu
> without using grant tables and without domu access to that page causing
> a vmexit?
Pick a reserved page in guest''s physical memory map. Communicate that
address to both domU and qemu somehow (e.g., one might pick it and
communicate it to the other; or it may be a statically assigned well-known
address; or whatever). Qemu can map that page, as obviously can the guest.

To get that reserved page, you could: (1) pick an unused frame slot in the
hole below 4GB (e.g., between LAPIC and IOAPIC mappings) allocate a page
there from qemu, or hvmloader, or domU, or...; or (2) you could have hvm
domain builder, or hvmloader, reserve a page from the guest''s regular
RAM
space, and update guest e820 appropiately to indicate page is reserved.
Communicate that address as necessary, and away you go.

 -- Keir
> To set up a custom kernel debug dll under windows I need to load very
> early, much earlier than my PV drivers. To have any hope of usefully
> increasing the speed over serial port debugging I need to be able to
> transfer data faster than 1-byte-per-vmexit...
> 
> Thanks
> 
> James
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


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> 
> > Is it possible to share a page of memory between qemu in dom0 and
domu
> > without using grant tables and without domu access to that page
causing
> > a vmexit?
> 
> Pick a reserved page in guest''s physical memory map. Communicate
that
> address to both domU and qemu somehow (e.g., one might pick it and
> communicate it to the other; or it may be a statically assigned
well-known
> address; or whatever). Qemu can map that page, as obviously can the
guest.
> 
> To get that reserved page, you could: (1) pick an unused frame slot in
the
> hole below 4GB (e.g., between LAPIC and IOAPIC mappings) allocate a
page
> there from qemu, or hvmloader, or domU, or...; or (2) you could have
hvm
> domain builder, or hvmloader, reserve a page from the guest''s
regular
RAM
> space, and update guest e820 appropiately to indicate page is
reserved.
> Communicate that address as necessary, and away you go.
> 
I''m still not sure I understand it all... might the following work
them:
1. Allocate a page of memory in my windows code
2. Write the PFN to a known io port (with some sort of interlock so that
a random io write that hits that port isn''t going to do anything bad)
3. qemu is called when the io port is triggered and maps the page to its
own space

Can qemu map an actual page of domu memory without a grant reference
(remember, grant table is not set up at this point)? Or do I need to map
a page of shared memory into the memory hole I allocated?

What calls should I be making in qemu to actually map the page?

Thanks

James

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On 17/10/2010 09:23, "James Harper"
<james.harper@bendigoit.com.au> wrote:
> I''m still not sure I understand it all... might the following work
them:
> 1. Allocate a page of memory in my windows code
> 2. Write the PFN to a known io port (with some sort of interlock so that
> a random io write that hits that port isn''t going to do anything
bad)
> 3. qemu is called when the io port is triggered and maps the page to its
> own space
Oh yes, that is probably the best way for you to get a page of memory.
> Can qemu map an actual page of domu memory without a grant reference
> (remember, grant table is not set up at this point)? Or do I need to map
> a page of shared memory into the memory hole I allocated?
Qemu can map domU memory with no need for a grant reference. Assuming you
allocated a page of RAM in step 1 above, you have not allocated a
''memory
hole'' and so there is no mapping into that pseudophysical frame slot to
be
done.
> What calls should I be making in qemu to actually map the page?
Probably ''xc_map_foreign_range(xc_handle, domid, XC_PAGE_SIZE,
PROT_READ|PROT_WRITE, pfn_from_your_windows_driver)''

 -- Keir



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