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

[Chris Wilson]

Quoting Zhenyu Wang (2020-10-19 10:19:09)
> 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?

Do we need it for gvt since the guest has no access to HW, so the host
should be doing all the vt'd w/a. (In particular, the scanout overfetch
causing the problems here.) E.g. in gvt, the guest framebuffer is
transported via magics to the host, and the host creates a GGTT entry
for it.

For detecting a hypervisor, following on from the KVM_CPUID_SIGNATURE
hint, we find arch/x86 does detect_hypervisor_platform() at load which
populates EXPORT_SYMBOL(x86_hyper_type)

So something like,

#include <asm/hypervisor.h>
@@ -1764,7 +1766,7 @@ static inline bool intel_vtd_active(void)
        if (intel_iommu_gfx_mapped)
                return true;
 #endif
-       return false;
+       return x86_hyper_type != X86_HYPER_NATIVE;
 }

should work.
-Chris