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>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 8e0c0844c6b9681e31e64bdeba0f1822108bec3e..d1a114d8d22b7f2c797883784f8211c202e5dafe 100644 (file)
--- 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);