Better reflect the structure of the code and metion why we could not
always honor the guest.
Signed-off-by: Chia-I Wu <olva...@gmail.com>
Cc: Gurchetan Singh <gurchetansi...@chromium.org>
Cc: Gerd Hoffmann <kra...@redhat.com>
---
arch/x86/kvm/vmx/vmx.c | 27 +++++++++++++++++----------
1 file changed, 17 insertions(+), 10 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3be25ecae145..266ef87042da 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6854,17 +6854,24 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t
gfn, bool is_mmio)
u8 cache;
u64 ipat = 0;
- /* For VT-d and EPT combination
- * 1. MMIO: always map as UC
- * 2. EPT with VT-d:
- * a. VT-d without snooping control feature: can't guarantee the
- * result, try to trust guest.
- * b. VT-d with snooping control feature: snooping control feature of
- * VT-d engine can guarantee the cache correctness. Just set it
- * to WB to keep consistent with host. So the same as item 3.
- * 3. EPT without VT-d: always map as WB and set IPAT=1 to keep
- * consistent with host MTRR
+ /* We wanted to honor guest CD/MTRR/PAT, but doing so could result in
+ * memory aliases with conflicting memory types and sometimes MCEs.
+ * We have to be careful as to what are honored and when.
+ *
+ * For MMIO, guest CD/MTRR are ignored. The EPT memory type is set to
+ * UC. The effective memory type is UC or WC depending on guest PAT.
+ * This was historically the source of MCEs and we want to be
+ * conservative.
+ *
+ * When there is no need to deal with noncoherent DMA (e.g., no VT-d
+ * or VT-d has snoop control), guest CD/MTRR/PAT are all ignored. The
+ * EPT memory type is set to WB. The effective memory type is forced
+ * WB.
+ *
+ * Otherwise, we trust guest. Guest CD/MTRR/PAT are all honored. The
+ * EPT memory type is used to emulate guest CD/MTRR.
*/
+
if (is_mmio) {
cache = MTRR_TYPE_UNCACHABLE;
goto exit;
--
2.25.0.265.gbab2e86ba0-goog
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel
