[PATCH] drm/i915: Do not enable movntdqa optimization in hypervisor guest

[Chris Wilson]

Quoting changbin.du at intel.com (2017-12-22 06:31:49)
> From: Changbin Du <changbin.du at intel.com>
> 
> 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.)
> 
> This patch try not to enable movntdqa optimization if the driver is
> running in hypervisor guest.
> 
> Signed-off-by: Changbin Du <changbin.du at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_memcpy.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_memcpy.c b/drivers/gpu/drm/i915/i915_memcpy.c
> index 49a0794..8921f40 100644
> --- a/drivers/gpu/drm/i915/i915_memcpy.c
> +++ b/drivers/gpu/drm/i915/i915_memcpy.c
> @@ -96,6 +96,11 @@ bool i915_memcpy_from_wc(void *dst, const void *src, unsigned long len)
>  
>  void i915_memcpy_init_early(struct drm_i915_private *dev_priv)
>  {
> -       if (static_cpu_has(X86_FEATURE_XMM4_1))
> +       /**

This isn't a kerneldoc, so just /* for a regular comment.

> +        * Some hypervisors (e.g. KVM) don't support VEX-prefix instructions
> +        * emulation. So don't enable movntdqa in hypervisor guest.
> +        */
> +       if (static_cpu_has(X86_FEATURE_XMM4_1) &&
> +           !boot_cpu_has(X86_FEATURE_HYPERVISOR))
>                 static_branch_enable(&has_movntdqa);

Code checks out, and I believe you that some hypervisor setups may trap
those instructions, so

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

We could try and jigger the init order about, but we do need this setup
quite early so that we use the knowledge in other setup. Considering
that, an explicit check for a hypervisor seems sensible.
-Chris