[Intel-gfx] [PATCH v2] drm/i915: Do not enable movntdqa optimization in hypervisor guest

[changbin.du at intel.com]

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.

v2: update comment.

Signed-off-by: Changbin Du <changbin.du at intel.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 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..79f8ec7 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))
+	/*
+	 * 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);
 }
-- 
2.7.4