[Intel-gfx] drm/i915: Detecting Vt-d when running as guest os

[Zhenyu Wang]

On 2020.10.16 17:19:19 +0200, Stefan Fritsch wrote:
> Hi,
> 
> if Linux is running as a guest and the host is doing igd-pass-thorugh with 
> VT-d enabled, the i915 driver does not work all that great. The most 
> obvious problem is that there are dozens of 'Fault errors on pipe A' 
> errrors logged per second, but depending on the hardware there can be 
> other issues, too. I will send a patch to rate-limit that message in a 
> separate mail.
> 
> The i915 has various quirks for VT-d and these should be enabled even if 
> Linux is running as a guest and does itself have iommu enabled. I have 
> checked that making intel_vtd_active() form i915_drv.h return true makes 
> the error messages go away.  How could Linux detect this situation? Maybe 
> simply check the Hypervisor cpuid bit? Or would you prefer a module 
> parameter, or a combination of both? Or is there another way to detect 
> that VT-d is enabled for the igd device?
> 

I think that's right, although I haven't tried to force intel_vtd_active()
for guest, but I did see those fault errors on some machine. You can use
hypervisor cpuid bit, and need to seperate case for GVT which is detected by
intel_vgpu_active(), but I'm not sure if this should be taken in nested case,
suppose those quirks should still work?

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