From 6c7b2202e4d11572ab23a89aeec49005b94bb966 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Thu, 17 Nov 2022 12:25:02 -0500 Subject: KVM: x86: avoid memslot check in NX hugepage recovery if it cannot succeed Since gfn_to_memslot() is relatively expensive, it helps to skip it if it the memslot cannot possibly have dirty logging enabled. In order to do this, add to struct kvm a counter of the number of log-page memslots. While the correct value can only be read with slots_lock taken, the NX recovery thread is content with using an approximate value. Therefore, the counter is an atomic_t. Based on https://lore.kernel.org/kvm/20221027200316.2221027-2-dmatlack@google.com/ by David Matlack. Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu/mmu.c | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) (limited to 'arch/x86/kvm/mmu') diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index cfff74685a25..4736d7849c60 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -6878,16 +6878,32 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm) WARN_ON_ONCE(!sp->nx_huge_page_disallowed); WARN_ON_ONCE(!sp->role.direct); - slot = gfn_to_memslot(kvm, sp->gfn); - WARN_ON_ONCE(!slot); - /* * Unaccount and do not attempt to recover any NX Huge Pages * that are being dirty tracked, as they would just be faulted * back in as 4KiB pages. The NX Huge Pages in this slot will be * recovered, along with all the other huge pages in the slot, * when dirty logging is disabled. + * + * Since gfn_to_memslot() is relatively expensive, it helps to + * skip it if it the test cannot possibly return true. On the + * other hand, if any memslot has logging enabled, chances are + * good that all of them do, in which case unaccount_nx_huge_page() + * is much cheaper than zapping the page. + * + * If a memslot update is in progress, reading an incorrect value + * of kvm->nr_memslots_dirty_logging is not a problem: if it is + * becoming zero, gfn_to_memslot() will be done unnecessarily; if + * it is becoming nonzero, the page will be zapped unnecessarily. + * Either way, this only affects efficiency in racy situations, + * and not correctness. */ + slot = NULL; + if (atomic_read(&kvm->nr_memslots_dirty_logging)) { + slot = gfn_to_memslot(kvm, sp->gfn); + WARN_ON_ONCE(!slot); + } + if (slot && kvm_slot_dirty_track_enabled(slot)) unaccount_nx_huge_page(kvm, sp); else if (is_tdp_mmu_page(sp)) -- cgit v1.2.3