Re: [PATCH] KVM: x86: SVM: fix nested PAUSE filtering

[Paolo Bonzini]

On 5/18/22 09:27, Maxim Levitsky wrote:
> To fix this, change the fallback strategy - ignore the guest threshold
> values, but use/update the host threshold values, instead of using zeros.

Hmm, now I remember why it was using the guest values.  It's because, if
the L1 hypervisor specifies COUNT=0 or does not have filtering enabled,
we need to obey and inject a vmexit on every PAUSE.  So something like this:

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index f209c1ca540c..e6153fd3ae47 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -616,6 +616,8 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
 	struct kvm_vcpu *vcpu = &svm->vcpu;
 	struct vmcb *vmcb01 = svm->vmcb01.ptr;
 	struct vmcb *vmcb02 = svm->nested.vmcb02.ptr;
+	u32 pause_count12;
+	u32 pause_thresh12;
 
 	/*
 	 * Filled at exit: exit_code, exit_code_hi, exit_info_1, exit_info_2,
@@ -671,20 +673,25 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
 	if (!nested_vmcb_needs_vls_intercept(svm))
 		vmcb02->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
 
+	pause_count12 = svm->pause_filter_enabled ? svm->nested.ctl.pause_filter_count : 0;
+	pause_thresh12 = svm->pause_threshold_enabled ? svm->nested.ctl.pause_filter_thresh : 0;
 	if (kvm_pause_in_guest(svm->vcpu.kvm)) {
-		/* use guest values since host doesn't use them */
-		vmcb02->control.pause_filter_count =
-				svm->pause_filter_enabled ?
-				svm->nested.ctl.pause_filter_count : 0;
-
-		vmcb02->control.pause_filter_thresh =
-				svm->pause_threshold_enabled ?
-				svm->nested.ctl.pause_filter_thresh : 0;
+		/* use guest values since host doesn't intercept PAUSE */
+		vmcb02->control.pause_filter_count = pause_count12;
+		vmcb02->control.pause_filter_thresh = pause_thresh12;
 
 	} else {
-		/* use host values otherwise */
+		/* start from host values otherwise */
 		vmcb02->control.pause_filter_count = vmcb01->control.pause_filter_count;
 		vmcb02->control.pause_filter_thresh = vmcb01->control.pause_filter_thresh;
+
+		/* ... but ensure filtering is disabled if so requested.  */
+		if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_PAUSE)) {
+			if (!pause_count12)
+				vmcb02->control.pause_filter_count = 0;
+			if (!pause_thresh12)
+				vmcb02->control.pause_filter_thresh = 0;
+		}
 	}
 
 	nested_svm_transition_tlb_flush(vcpu);


What do you think?