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[52.49.196.232]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-477b103d312sm706385e9.13.2025.11.18.09.11.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Nov 2025 09:11:19 -0800 (PST) From: griffoul@gmail.com X-Google-Original-From: griffoul@gmail.org To: kvm@vger.kernel.org Cc: seanjc@google.com, pbonzini@redhat.com, vkuznets@redhat.com, shuah@kernel.org, dwmw@amazon.co.uk, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Fred Griffoul Subject: [PATCH v2 01/10] KVM: nVMX: Implement cache for L1 MSR bitmap Date: Tue, 18 Nov 2025 17:11:04 +0000 Message-ID: <20251118171113.363528-2-griffoul@gmail.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251118171113.363528-1-griffoul@gmail.org> References: <20251118171113.363528-1-griffoul@gmail.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Fred Griffoul 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 --- 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, str= uct loaded_vmcs *vmcs) vcpu->arch.regs_dirty =3D 0; } =20 +/* + * 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 !=3D gpa)) { + read_unlock(&gpc->lock); + err =3D 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 =3D to_vmx(vcpu); @@ -344,6 +372,9 @@ static void free_nested(struct kvm_vcpu *vcpu) vmx->nested.vmxon =3D false; vmx->nested.smm.vmxon =3D false; vmx->nested.vmxon_ptr =3D INVALID_GPA; + + kvm_gpc_deactivate(&vmx->nested.msr_bitmap_cache); + free_vpid(vmx->nested.vpid02); vmx->nested.posted_intr_nv =3D -1; vmx->nested.current_vmptr =3D 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 =3D vmx->nested.vmcs02.msr_bitmap; - struct kvm_host_map map; + struct gfn_to_pfn_cache *gpc; =20 /* 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(stru= ct kvm_vcpu *vcpu, return true; } =20 - if (kvm_vcpu_map_readonly(vcpu, gpa_to_gfn(vmcs12->msr_bitmap), &map)) + gpc =3D &vmx->nested.msr_bitmap_cache; + if (nested_gpc_lock(gpc, vmcs12->msr_bitmap)) return false; =20 - msr_bitmap_l1 =3D (unsigned long *)map.hva; + msr_bitmap_l1 =3D (unsigned long *)gpc->khva; =20 /* * 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); =20 - kvm_vcpu_unmap(vcpu, &map); + nested_gpc_unlock(gpc); =20 vmx->nested.force_msr_bitmap_recalc =3D false; =20 @@ -5490,6 +5522,8 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu) =20 vmx->nested.vpid02 =3D allocate_vpid(); =20 + kvm_gpc_init(&vmx->nested.msr_bitmap_cache, vcpu->kvm); + vmx->nested.vmcs02_initialized =3D false; vmx->nested.vmxon =3D true; =20 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 { =20 struct loaded_vmcs vmcs02; =20 + 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. --=20 2.43.0