linux-kernel - [PATCH v2 31/32] KVM: x86: Simplify userspace filter logic when disabling MSR interception

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Message-ID: <20250610225737.156318-32-seanjc@google.com>
Date: Tue, 10 Jun 2025 15:57:36 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chao Gao <chao.gao@...el.com>, Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Francesco Lavra <francescolavra.fl@...il.com>, 
	Manali Shukla <Manali.Shukla@....com>
Subject: [PATCH v2 31/32] KVM: x86: Simplify userspace filter logic when
 disabling MSR interception

Refactor {svm,vmx}_disable_intercept_for_msr() to simplify the handling of
userspace filters that disallow access to an MSR.  The more complicated
logic is no longer needed or justified now that KVM recalculates all MSR
intercepts on a userspace MSR filter change, i.e. now that KVM doesn't
need to also update shadow bitmaps.

No functional change intended.

Suggested-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 24 ++++++++++--------------
 arch/x86/kvm/vmx/vmx.c | 24 ++++++++++--------------
 2 files changed, 20 insertions(+), 28 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e3c49c763225..5453478d1ca3 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -691,24 +691,20 @@ void svm_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
 	void *msrpm = svm->msrpm;
 
 	/* Don't disable interception for MSRs userspace wants to handle. */
-	if ((type & MSR_TYPE_R) &&
-	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ)) {
-		svm_set_msr_bitmap_read(msrpm, msr);
-		type &= ~MSR_TYPE_R;
+	if (type & MSR_TYPE_R) {
+		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ))
+			svm_clear_msr_bitmap_read(msrpm, msr);
+		else
+			svm_set_msr_bitmap_read(msrpm, msr);
 	}
 
-	if ((type & MSR_TYPE_W) &&
-	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE)) {
-		svm_set_msr_bitmap_write(msrpm, msr);
-		type &= ~MSR_TYPE_W;
+	if (type & MSR_TYPE_W) {
+		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE))
+			svm_clear_msr_bitmap_write(msrpm, msr);
+		else
+			svm_set_msr_bitmap_write(msrpm, msr);
 	}
 
-	if (type & MSR_TYPE_R)
-		svm_clear_msr_bitmap_read(msrpm, msr);
-
-	if (type & MSR_TYPE_W)
-		svm_clear_msr_bitmap_write(msrpm, msr);
-
 	svm_hv_vmcb_dirty_nested_enlightenments(vcpu);
 	svm->nested.force_msr_bitmap_recalc = true;
 }
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index bdff81f8288d..277c6b5b5d5f 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3962,23 +3962,19 @@ void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
 
 	vmx_msr_bitmap_l01_changed(vmx);
 
-	if ((type & MSR_TYPE_R) &&
-	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ)) {
-		vmx_set_msr_bitmap_read(msr_bitmap, msr);
-		type &= ~MSR_TYPE_R;
+	if (type & MSR_TYPE_R) {
+		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ))
+			vmx_clear_msr_bitmap_read(msr_bitmap, msr);
+		else
+			vmx_set_msr_bitmap_read(msr_bitmap, msr);
 	}
 
-	if ((type & MSR_TYPE_W) &&
-	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE)) {
-		vmx_set_msr_bitmap_write(msr_bitmap, msr);
-		type &= ~MSR_TYPE_W;
+	if (type & MSR_TYPE_W) {
+		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE))
+			vmx_clear_msr_bitmap_write(msr_bitmap, msr);
+		else
+			vmx_set_msr_bitmap_write(msr_bitmap, msr);
 	}
-
-	if (type & MSR_TYPE_R)
-		vmx_clear_msr_bitmap_read(msr_bitmap, msr);
-
-	if (type & MSR_TYPE_W)
-		vmx_clear_msr_bitmap_write(msr_bitmap, msr);
 }
 
 void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
-- 
2.50.0.rc0.642.g800a2b2222-goog