Re: [PATCH] vfio/igd: Check host PCI address when probing

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 17 Apr 2025 01:41:22 +0800
Tomita Moeko <tomitamoeko@gmail.com> wrote:

> On 4/17/25 00:10, Alex Williamson wrote:
> > On Wed, 16 Apr 2025 23:45:08 +0800
> > Tomita Moeko <tomitamoeko@gmail.com> wrote:
> >   
> >> On 4/16/25 03:04, Alex Williamson wrote:  
> >>> On Wed, 16 Apr 2025 01:36:15 +0800
> >>> Tomita Moeko <tomitamoeko@gmail.com> wrote:    
> >>>>
> >>>> The generation register also exists on discrete GPUs. In the new xe
> >>>> driver [1], the Battlemage discrete GPU shares the same logic reading
> >>>> GMD_ID_DISPLAY register. The driver itself uses is_dgfx bit mapped to
> >>>> device id. In QEMU, we need to know whether the device is a supported
> >>>> IGD device first before applying the IGD-specific quirk, especially
> >>>> for legacy mode.
> >>>>
> >>>> The most feasible way is to check if kernel exposes VFIO_REGION_SUBTYPE_
> >>>> INTEL_IGD_OPREGION on that device I think, as only IGD has OpRegion.
> >>>>
> >>>> i915 driver [2] and Arrow Lake datasheet [3] shows that Intel has
> >>>> removed the BDSM register by making the DSM range part of BAR2 since
> >>>> Meteor Lake and onwards. QEMU only need to quirk on the register for
> >>>> IGD devices until Raptor Lake, meaning that the device list is fixed
> >>>> for now.
> >>>>
> >>>> By the way, for legacy mode, I think we should only support it until
> >>>> Gen 9, as Intel only provide VBIOS or CSM support until that generation,
> >>>> and seabios cannot handle 64 bit BDSM register. I'm also wondering if
> >>>> VGA really works on newer generations.    
> >>>
> >>> If it's a VGA class device, it really should, but without CSM I could
> >>> see why you have doubts.    
> >>
> >> Without CSM/VBIOS there is no pre-boot video, but when it booted to OS,
> >> driver is used for video rather than VGA. Though it claims itself as
> >> VGA class, it does not have VGA compatiblity. A770 even does not have
> >> IO BAR, definitely it cannot handle VGA decoding.  
> > 
> > VGA ranges are implicit in a VGA class device, they're not backed by
> > BARs.  Lack of CSM support makes it more difficult to prove whether VGA
> > support is present since we can't easily initialize the device for a
> > legacy OS, but I don't think lack of CSM necessary proves the hardware
> > doesn't support VGA.  If we really cared, we could probably do some low
> > level experiments writing into the VGA frame buffer range to test if
> > it's present and behaves as expected relative to memory and IO enable
> > bits.  
> 
> Sorry for my misunderstanding. The bridge control register in PCI bridge
> config space determines forwarding VGA IO/MMIO accesses to which device,
> BAR is not related in this process.
> 
> As device initialization (by legacy VBIOS) is required for booting
> legacy OS with seabios, limiting legacy mode to gen9 and older sounds
> resonable.
> 
> Trying VGA framebuffer would be difficult we have to disable EFI GOP and
> manually instruct the device to enter VGA mode without VBIOS routines.
> It's not a major problem here, let's skip it for now?

This is only a curiosity as far as I'm concerned, I agree with your
proposal that we can drop legacy mode where the bare metal system
doesn't even support CSM.  Thanks,

Alex