KVM: svm: handle KVM_X86_QUIRK_CD_NW_CLEARED in svm_get_mt_mask

We can disable CD unconditionally when there is no assigned device.
KVM now forces guest PAT to all-writeback in that case, so it makes
sense to also force CR0.CD=0.

When there are assigned devices, emulate cache-disabled operation
through the page tables.  This behavior is consistent with VMX
microcode, where CD/NW are not touched by vmentry/vmexit.  However,
keep this dependent on the quirk because OVMF enables the caches
too late.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 8 insertions, 7 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 8e0c0844c6b9..d1a114d8d22b 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1173,6 +1173,10 @@ static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
         if (!is_mmio && !kvm_arch_has_assigned_device(vcpu->kvm))
                 return 0;
 
+        if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED) &&
+            kvm_read_cr0(vcpu) & X86_CR0_CD)
+                return _PAGE_NOCACHE;
+
         mtrr = kvm_mtrr_get_guest_memory_type(vcpu, gfn);
         return mtrr2protval[mtrr];
 }
@@ -1667,13 +1671,10 @@ static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 
         if (!vcpu->fpu_active)
                 cr0 |= X86_CR0_TS;
-        /*
-         * re-enable caching here because the QEMU bios
-         * does not do it - this results in some delay at
-         * reboot
-         */
-        if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
-                cr0 &= ~(X86_CR0_CD | X86_CR0_NW);
+
+        /* These are emulated via page tables.  */
+        cr0 &= ~(X86_CR0_CD | X86_CR0_NW);
+
         svm->vmcb->save.cr0 = cr0;
         mark_dirty(svm->vmcb, VMCB_CR);
         update_cr0_intercept(svm);