From c6cecc4b9324b97775d7002a13460c247f586e8e Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 18 Aug 2021 23:56:15 +0000 Subject: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU Make a final call to direct_pte_prefetch_many() if there are "trailing" SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN. The call to direct_pte_prefetch_many() in the loop only handles the case where there are !PRESENT SPTEs preceding a PRESENT SPTE. E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all SPTEs for the following GFNs are !PRESENT, the loop will terminate with "start = sptep+1" and not prefetch any SPTEs. Prefetching trailing SPTEs as intended can drastically reduce the number of guest page faults, e.g. accessing the first byte of every 4kb page in a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory, the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M. Note, this only affects memory that is backed by 4kb pages as KVM doesn't prefetch when installing hugepages. Shadow paging prefetching is not affected as it does not batch the prefetches due to the need to process the corresponding guest PTE. The TDP MMU is not affected because it doesn't have prefetching, yet... Fixes: 957ed9effd80 ("KVM: MMU: prefetch ptes when intercepted guest #PF") Cc: Sergey Senozhatsky Cc: Ben Gardon Signed-off-by: Sean Christopherson Message-Id: <20210818235615.2047588-1-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu/mmu.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 1a64ba5b9437..4238fe3e91c2 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2842,11 +2842,13 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu, if (!start) continue; if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0) - break; + return; start = NULL; } else if (!start) start = spte; } + if (start) + direct_pte_prefetch_many(vcpu, sp, start, spte); } static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)