linux-kernel - [PATCH v2 01/10] KVM: nVMX: Implement cache for L1 MSR bitmap

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Message-ID: <20251118171113.363528-2-griffoul@gmail.org>
Date: Tue, 18 Nov 2025 17:11:04 +0000
From: griffoul@...il.com
To: kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	pbonzini@...hat.com,
	vkuznets@...hat.com,
	shuah@...nel.org,
	dwmw@...zon.co.uk,
	linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Fred Griffoul <fgriffo@...zon.co.uk>
Subject: [PATCH v2 01/10] KVM: nVMX: Implement cache for L1 MSR bitmap

From: Fred Griffoul <fgriffo@...zon.co.uk>

Introduce a gfn_to_pfn_cache to optimize L1 MSR bitmap access by
replacing map/unmap operations. This optimization reduces overhead
during L2 VM-entry where nested_vmx_prepare_msr_bitmap() merges L1's MSR
intercepts with L0's requirements.

Current implementation using kvm_vcpu_map_readonly() and
kvm_vcpu_unmap() creates significant performance impact, mostly with
unmanaged guest memory.

The cache is initialized when entering VMX operation and deactivated
when VMX operation ends.

Signed-off-by: Fred Griffoul <fgriffo@...zon.co.uk>
---
 arch/x86/kvm/vmx/nested.c | 42 +++++++++++++++++++++++++++++++++++----
 arch/x86/kvm/vmx/vmx.h    |  2 ++
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 8b131780e981..0de84b30c41d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -315,6 +315,34 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
 	vcpu->arch.regs_dirty = 0;
 }
 
+/*
+ * Maps a single guest page starting at @gpa and lock the cache for access.
+ */
+static int nested_gpc_lock(struct gfn_to_pfn_cache *gpc, gpa_t gpa)
+{
+	int err;
+
+	if (!PAGE_ALIGNED(gpa))
+		return -EINVAL;
+retry:
+	read_lock(&gpc->lock);
+	if (!kvm_gpc_check(gpc, PAGE_SIZE) || (gpc->gpa != gpa)) {
+		read_unlock(&gpc->lock);
+		err = kvm_gpc_activate(gpc, gpa, PAGE_SIZE);
+		if (err)
+			return err;
+
+		goto retry;
+	}
+
+	return 0;
+}
+
+static void nested_gpc_unlock(struct gfn_to_pfn_cache *gpc)
+{
+	read_unlock(&gpc->lock);
+}
+
 static void nested_put_vmcs12_pages(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -344,6 +372,9 @@ static void free_nested(struct kvm_vcpu *vcpu)
 	vmx->nested.vmxon = false;
 	vmx->nested.smm.vmxon = false;
 	vmx->nested.vmxon_ptr = INVALID_GPA;
+
+	kvm_gpc_deactivate(&vmx->nested.msr_bitmap_cache);
+
 	free_vpid(vmx->nested.vpid02);
 	vmx->nested.posted_intr_nv = -1;
 	vmx->nested.current_vmptr = INVALID_GPA;
@@ -625,7 +656,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 	int msr;
 	unsigned long *msr_bitmap_l1;
 	unsigned long *msr_bitmap_l0 = vmx->nested.vmcs02.msr_bitmap;
-	struct kvm_host_map map;
+	struct gfn_to_pfn_cache *gpc;
 
 	/* Nothing to do if the MSR bitmap is not in use.  */
 	if (!cpu_has_vmx_msr_bitmap() ||
@@ -648,10 +679,11 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 			return true;
 	}
 
-	if (kvm_vcpu_map_readonly(vcpu, gpa_to_gfn(vmcs12->msr_bitmap), &map))
+	gpc = &vmx->nested.msr_bitmap_cache;
+	if (nested_gpc_lock(gpc, vmcs12->msr_bitmap))
 		return false;
 
-	msr_bitmap_l1 = (unsigned long *)map.hva;
+	msr_bitmap_l1 = (unsigned long *)gpc->khva;
 
 	/*
 	 * To keep the control flow simple, pay eight 8-byte writes (sixteen
@@ -739,7 +771,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
 					 MSR_IA32_PL3_SSP, MSR_TYPE_RW);
 
-	kvm_vcpu_unmap(vcpu, &map);
+	nested_gpc_unlock(gpc);
 
 	vmx->nested.force_msr_bitmap_recalc = false;
 
@@ -5490,6 +5522,8 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
 
 	vmx->nested.vpid02 = allocate_vpid();
 
+	kvm_gpc_init(&vmx->nested.msr_bitmap_cache, vcpu->kvm);
+
 	vmx->nested.vmcs02_initialized = false;
 	vmx->nested.vmxon = true;
 
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index ea93121029f9..d76621403c28 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -152,6 +152,8 @@ struct nested_vmx {
 
 	struct loaded_vmcs vmcs02;
 
+	struct gfn_to_pfn_cache msr_bitmap_cache;
+
 	/*
 	 * Guest pages referred to in the vmcs02 with host-physical
 	 * pointers, so we must keep them pinned while L2 runs.
-- 
2.43.0