Re: [PATCH] xen/pt: fix igd passthrough for pc machine with xen accelerator

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

On 5/17/2023 2:39 AM, Michael Tokarev wrote:
> 08.02.2023 05:03, Chuck Zmudzinski wrote:
> > Commit 998250e97661 ("xen, gfx passthrough: register host bridge specific
> > to passthrough") uses the igd-passthrough-i440FX pci host device with
> > the xenfv machine type and igd-passthru=on, but using it for the pc
> > machine type, xen accelerator, and igd-passtru=on was omitted from that
> > commit.
> > 
> > The igd-passthru-i440FX pci host device is also needed for guests
> > configured with the pc machine type, the xen accelerator, and
> > igd-passthru=on. Specifically, tests show that not using the igd-specific
> > pci host device with the Intel igd passed through to the guest results
> > in slower startup performance and reduced resolution of the display
> > during startup. This patch fixes this issue.
> > 
> > To simplify the logic that is needed to support both the --enable-xen
> > and the --disable-xen configure options, introduce the boolean symbol
> > pc_xen_igd_gfx_pt_enabled() whose value is set appropriately in the
> > sysemu/xen.h header file as the test to determine whether or not
> > to use the igd-passthrough-i440FX pci host device instead of the
> > normal i440FX pci host device.
> > 
> > Fixes: 998250e97661 ("xen, gfx passthrough: register host bridge specific 
> > to passthrough")
> > Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
>
> Has this change been forgotten?  Is it not needed anymore?

Short answer:

After 4f67543b ("xen/pt: reserve PCI slot 2 for Intel igd-passthru ") was
applied, I was inclined to think this change is not needed anymore, but
it would not hurt to add this change also, and now I think it might be
more correct to also add this change.

Longer explanation:

I strongly desired that at least one of the patches I proposed to improve
support for Intel IGD passthrough with xen be committed. Since
4f67543b ("xen/pt: reserve PCI slot 2 for Intel igd-passthru ") that fixed
Intel IGD passthrough for the xenfv machine type has been committed,
I reasoned that there is not such a great need to also fix Intel IGD
passthrough for the pc machine type with xen so I did not push hard for
this patch to also be applied.

My requirement was that either the xenfv machine be fixed or the pc
machine be fixed. I did not think it was necessary to fix them both, and
4f67543b ("xen/pt: reserve PCI slot 2 for Intel igd-passthru ") fixed the
xenfv machine. But this patch provides the additional fix for the pc machine,
a fix that is distinct from the fix that has already been committed for the
xenfv machine, and it probably should also be applied so pc and xenfv
machines will work equally well with Intel IGD passthrough.

In other words, it is good to fix at least one of the two broken machines 
configured
for Intel IGD passthrough and xen, it is better to fix them both. We already 
fixed
one of them with 4f67543b ("xen/pt: reserve PCI slot 2 for Intel igd-passthru 
"),
this patch would fix the other one.

If you want to add this change also, let's make sure recent changes to the
xen header files do not require the patch to be rebased before committing
it.

Chuck