From 127037591c84e8d550e8951d274a5e291d3ed56f Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 23 Sep 2020 15:04:25 -0700 Subject: [PATCH] KVM: x86/mmu: Bail early from final #PF handling on spurious faults Detect spurious page faults, e.g. page faults that occur when multiple vCPUs simultaneously access a not-present page, and skip the SPTE write, prefetch, and stats update for spurious faults. Note, the performance benefits of skipping the write and prefetch are likely negligible, and the false positive stats adjustment is probably lost in the noise. The primary motivation is to play nice with TDX's SEPT in the long term. SEAMCALLs (to program SEPT entries) are quite costly, e.g. thousands of cycles, and a spurious SEPT update will result in a SEAMCALL error (which KVM will ideally treat as fatal). Reported-by: Kai Huang Signed-off-by: Sean Christopherson Message-Id: <20200923220425.18402-5-sean.j.christopherson@intel.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu/mmu.c | 17 ++++++++++++++++- arch/x86/kvm/mmu/paging_tmpl.h | 3 +++ 2 files changed, 19 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 2360d33a27b1..7ec3d057e8e0 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2985,6 +2985,7 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn) /* Bits which may be returned by set_spte() */ #define SET_SPTE_WRITE_PROTECTED_PT BIT(0) #define SET_SPTE_NEED_REMOTE_TLB_FLUSH BIT(1) +#define SET_SPTE_SPURIOUS BIT(2) static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, unsigned int pte_access, int level, @@ -3073,7 +3074,9 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, spte = mark_spte_for_access_track(spte); set_pte: - if (mmu_spte_update(sptep, spte)) + if (*sptep == spte) + ret |= SET_SPTE_SPURIOUS; + else if (mmu_spte_update(sptep, spte)) ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH; return ret; } @@ -3128,6 +3131,15 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep, if (unlikely(is_mmio_spte(*sptep))) ret = RET_PF_EMULATE; + /* + * The fault is fully spurious if and only if the new SPTE and old SPTE + * are identical, and emulation is not required. + */ + if ((set_spte_ret & SET_SPTE_SPURIOUS) && ret == RET_PF_FIXED) { + WARN_ON_ONCE(!was_rmapped); + return RET_PF_SPURIOUS; + } + pgprintk("%s: setting spte %llx\n", __func__, *sptep); trace_kvm_mmu_set_spte(level, gfn, sptep); if (!was_rmapped && is_large_pte(*sptep)) @@ -3364,6 +3376,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t gpa, int write, ret = mmu_set_spte(vcpu, it.sptep, ACC_ALL, write, level, base_gfn, pfn, prefault, map_writable); + if (ret == RET_PF_SPURIOUS) + return ret; + direct_pte_prefetch(vcpu, it.sptep); ++vcpu->stat.pf_fixed; return ret; diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index e1066226b8f0..188ec2633bc5 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -711,6 +711,9 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gpa_t addr, ret = mmu_set_spte(vcpu, it.sptep, gw->pte_access, write_fault, it.level, base_gfn, pfn, prefault, map_writable); + if (ret == RET_PF_SPURIOUS) + return ret; + FNAME(pte_prefetch)(vcpu, gw, it.sptep); ++vcpu->stat.pf_fixed; return ret; -- 2.20.1