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

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Date:   Fri, 20 May 2022 16:05:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: Re: [PATCH] KVM: x86: SVM: fix nested PAUSE filtering

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?