KVM: Enable snooping control for supported hardware

Memory aliases with different memory type is a problem for guest. For the guest
without assigned device, the memory type of guest memory would always been the
same as host(WB); but for the assigned device, some part of memory may be used
as DMA and then set to uncacheable memory type(UC/WC), which would be a conflict of
host memory type then be a potential issue.

Snooping control can guarantee the cache correctness of memory go through the
DMA engine of VT-d.

[avi: fix build on ia64]

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 59b080c262e8..e8a5649f9c15 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3581,11 +3581,26 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 {
         u64 ret;
 
+        /* 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 IGMT=1 to keep
+         *    consistent with host MTRR
+         */
         if (is_mmio)
                 ret = MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
+        else if (vcpu->kvm->arch.iommu_domain &&
+                !(vcpu->kvm->arch.iommu_flags & KVM_IOMMU_CACHE_COHERENCY))
+                ret = kvm_get_guest_memory_type(vcpu, gfn) <<
+                      VMX_EPT_MT_EPTE_SHIFT;
         else
-                ret = (kvm_get_guest_memory_type(vcpu, gfn) <<
-                        VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IGMT_BIT;
+                ret = (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT)
+                        | VMX_EPT_IGMT_BIT;
 
         return ret;
 }