On Thu, 17 Apr 2025 01:41:22 +0800 Tomita Moeko <tomitamo...@gmail.com> wrote:
> On 4/17/25 00:10, Alex Williamson wrote: > > On Wed, 16 Apr 2025 23:45:08 +0800 > > Tomita Moeko <tomitamo...@gmail.com> wrote: > > > >> On 4/16/25 03:04, Alex Williamson wrote: > >>> On Wed, 16 Apr 2025 01:36:15 +0800 > >>> Tomita Moeko <tomitamo...@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
