From 50a417962a80525da54fa74105bcf17b479cd4bc Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 9 Jun 2021 16:42:28 -0700 Subject: [PATCH] KVM: nVMX: Consolidate VM-Enter/VM-Exit TLB flush and MMU sync logic Drop the dedicated nested_vmx_transition_mmu_sync() now that the MMU sync is handled via KVM_REQ_TLB_FLUSH_GUEST, and fold that flush into the all-encompassing nested_vmx_transition_tlb_flush(). Opportunistically add a comment explaning why nested EPT never needs to sync the MMU on VM-Enter. Signed-off-by: Sean Christopherson Message-Id: <20210609234235.1244004-9-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/vmx/nested.c | 87 +++++++++++++---------------------------------- 1 file changed, 23 insertions(+), 64 deletions(-) diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index d07b83b..8e2487f2 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -1063,48 +1063,6 @@ static void prepare_vmx_msr_autostore_list(struct kvm_vcpu *vcpu, } /* - * Returns true if the MMU needs to be sync'd on nested VM-Enter/VM-Exit. - * tl;dr: the MMU needs a sync if L0 is using shadow paging and L1 didn't - * enable VPID for L2 (implying it expects a TLB flush on VMX transitions). - * Here's why. - * - * If EPT is enabled by L0 a sync is never needed: - * - if it is disabled by L1, then L0 is not shadowing L1 or L2 PTEs, there - * cannot be unsync'd SPTEs for either L1 or L2. - * - * - if it is also enabled by L1, then L0 doesn't need to sync on VM-Enter - * VM-Enter as VM-Enter isn't required to invalidate guest-physical mappings - * (irrespective of VPID), i.e. L1 can't rely on the (virtual) CPU to flush - * stale guest-physical mappings for L2 from the TLB. And as above, L0 isn't - * shadowing L1 PTEs so there are no unsync'd SPTEs to sync on VM-Exit. - * - * If EPT is disabled by L0: - * - if VPID is enabled by L1 (for L2), the situation is similar to when L1 - * enables EPT: L0 doesn't need to sync as VM-Enter and VM-Exit aren't - * required to invalidate linear mappings (EPT is disabled so there are - * no combined or guest-physical mappings), i.e. L1 can't rely on the - * (virtual) CPU to flush stale linear mappings for either L2 or itself (L1). - * - * - however if VPID is disabled by L1, then a sync is needed as L1 expects all - * linear mappings (EPT is disabled so there are no combined or guest-physical - * mappings) to be invalidated on both VM-Enter and VM-Exit. - * - * Note, this logic is subtly different than nested_has_guest_tlb_tag(), which - * additionally checks that L2 has been assigned a VPID (when EPT is disabled). - * Whether or not L2 has been assigned a VPID by L0 is irrelevant with respect - * to L1's expectations, e.g. L0 needs to invalidate hardware TLB entries if L2 - * doesn't have a unique VPID to prevent reusing L1's entries (assuming L1 has - * been assigned a VPID), but L0 doesn't need to do a MMU sync because L1 - * doesn't expect stale (virtual) TLB entries to be flushed, i.e. L1 doesn't - * know that L0 will flush the TLB and so L1 will do INVVPID as needed to flush - * stale TLB entries, at which point L0 will sync L2's MMU. - */ -static bool nested_vmx_transition_mmu_sync(struct kvm_vcpu *vcpu) -{ - return !enable_ept && !nested_cpu_has_vpid(get_vmcs12(vcpu)); -} - -/* * Load guest's/host's cr3 at nested entry/exit. @nested_ept is true if we are * emulating VM-Entry into a guest with EPT enabled. On failure, the expected * Exit Qualification (for a VM-Entry consistency check VM-Exit) is assigned to @@ -1129,18 +1087,9 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, return -EINVAL; } - if (!nested_ept) { + if (!nested_ept) kvm_mmu_new_pgd(vcpu, cr3); - /* - * A TLB flush on VM-Enter/VM-Exit flushes all linear mappings - * across all PCIDs, i.e. all PGDs need to be synchronized. - * See nested_vmx_transition_mmu_sync() for more details. - */ - if (nested_vmx_transition_mmu_sync(vcpu)) - kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); - } - vcpu->arch.cr3 = cr3; kvm_register_mark_available(vcpu, VCPU_EXREG_CR3); @@ -1177,17 +1126,28 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx = to_vmx(vcpu); /* - * If VPID is disabled, linear and combined mappings are flushed on - * VM-Enter/VM-Exit, and guest-physical mappings are valid only for - * their associated EPTP. + * If vmcs12 doesn't use VPID, L1 expects linear and combined mappings + * for *all* contexts to be flushed on VM-Enter/VM-Exit, i.e. it's a + * full TLB flush from the guest's perspective. This is required even + * if VPID is disabled in the host as KVM may need to synchronize the + * MMU in response to the guest TLB flush. + * + * Note, using TLB_FLUSH_GUEST is correct even if nested EPT is in use. + * EPT is a special snowflake, as guest-physical mappings aren't + * flushed on VPID invalidations, including VM-Enter or VM-Exit with + * VPID disabled. As a result, KVM _never_ needs to sync nEPT + * entries on VM-Enter because L1 can't rely on VM-Enter to flush + * those mappings. */ - if (!enable_vpid) + if (!nested_cpu_has_vpid(vmcs12)) { + kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); return; + } + + /* L2 should never have a VPID if VPID is disabled. */ + WARN_ON(!enable_vpid); /* - * If vmcs12 doesn't use VPID, L1 expects linear and combined mappings - * for *all* contexts to be flushed on VM-Enter/VM-Exit. - * * If VPID is enabled and used by vmc12, but L2 does not have a unique * TLB tag (ASID), i.e. EPT is disabled and KVM was unable to allocate * a VPID for L2, flush the current context as the effective ASID is @@ -1199,13 +1159,12 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu, * * If a TLB flush isn't required due to any of the above, and vpid12 is * changing then the new "virtual" VPID (vpid12) will reuse the same - * "real" VPID (vpid02), and so needs to be sync'd. There is no direct + * "real" VPID (vpid02), and so needs to be flushed. There's no direct * mapping between vpid02 and vpid12, vpid02 is per-vCPU and reused for - * all nested vCPUs. + * all nested vCPUs. Remember, a flush on VM-Enter does not invalidate + * guest-physical mappings, so there is no need to sync the nEPT MMU. */ - if (!nested_cpu_has_vpid(vmcs12)) { - kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu); - } else if (!nested_has_guest_tlb_tag(vcpu)) { + if (!nested_has_guest_tlb_tag(vcpu)) { kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); } else if (is_vmenter && vmcs12->virtual_processor_id != vmx->nested.last_vpid) { -- 2.7.4