[Intel-gfx] Accelerated read from WC mem (i915_memcpy_from_wc()) may not work in virtualization world

[Chris Wilson]

Quoting Du, Changbin (2017-12-21 09:52:16)
> Hi Chris,
> Our QA reported a problem caused by movntdqa instructions. Currently, the KVM
> hypervisor doesn't support VEX-prefix instructions emulation. If users passthrough
> a GPU to guest with vfio option 'x-no-mmap=on', then all access to the BARs will
> be trapped and emulated. The KVM hypervisor would raise an inertal error to qemu
> which cause the guest killed. (Since 'movntdqa' ins is not supported.)
> 
> One possible solution is that disable this optimization at i915_memcpy_init_early.
> This require us to identify the CPUID to check if it is running in guest mode.

But do note we are already checking the CPUID/cpuflags for support; is
the hv not correcting them?
-Chris