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: NPaolo Bonzini <pbonzini@redhat.com>

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);