Xen devel - Jan 2009 - Mapping pci resources for graphics passthrough

Hi all,

I''ve been trying to understand the pci_setup function in hvmloader.c in
Xen
unstable. I''m kinda lost at Line 328.
base = (resource->base + bar_sz - 1) & ~(bar_sz - 1)

I''d appreciate if someone could explain to me why base is calculated
this
way. Specifically, I don''t understand the bitwise AND with the
one''s
complements of "bar_sz-1".

The reason why I''m doing this is that I''m hoping to modify
hvmloader such
that it can map the correct memory for a PCI-e video card. I''ve been
able to
passthrough the video card to a HVM guest. However, the OS (tried Linux,
Vista or XP) is not able to load the driver correctly, although lspci (on
Linux) does show that the video card has been passed through. I suspect it
is because the video RAM and ROM has not been mapped as required, thus the
video BIOS cannot be loaded. I would like to try out with only one guest
first, just to see if it works and leaving out the complexities of
multiplexing the graphics card with several guests. It is also fine if dom0
loses control of the graphics card. I can use serial.

Thanks and have a nice day!

Cheers,
Alex


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It¹s that way because a BAR base has to be aligned on a boundary the
power-of-two size of the BAR. Passing thru gfx before may not have worked
because the PCI I/O hole below 4GB was not big enough. That is fixed now in
xen-unstable by dynamically sizing the hole.

 -- Keir


On 22/01/2009 00:29, "Alexia Benington"
<alexbenington@gmail.com> wrote:
> I''ve been trying to understand the pci_setup function in
hvmloader.c in Xen
> unstable. I''m kinda lost at Line 328.
> base = (resource->base + bar_sz - 1) & ~(bar_sz - 1)
> 
> I''d appreciate if someone could explain to me why base is
calculated this way.
> Specifically, I don''t understand the bitwise AND with the
one''s complements of
> "bar_sz-1".
> 
> The reason why I''m doing this is that I''m hoping to
modify hvmloader such that
> it can map the correct memory for a PCI-e video card. I''ve been
able to
> passthrough the video card to a HVM guest. However, the OS (tried Linux,
Vista
> or XP) is not able to load the driver correctly, although lspci (on Linux)
> does show that the video card has been passed through. I suspect it is
because
> the video RAM and ROM has not been mapped as required, thus the video BIOS
> cannot be loaded. I would like to try out with only one guest first, just
to
> see if it works and leaving out the complexities of multiplexing the
graphics
> card with several guests. It is also fine if dom0 loses control of the
> graphics card. I can use serial.


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