From ecc5589f19a52e7e6501fe449047b19087ae11bb Mon Sep 17 00:00:00 2001 From: Marcelo Tosatti Date: Tue, 25 Nov 2008 15:58:07 +0100 Subject: [PATCH] KVM: MMU: optimize set_spte for page sync The write protect verification in set_spte is unnecessary for page sync. Its guaranteed that, if the unsync spte was writable, the target page does not have a write protected shadow (if it had, the spte would have been write protected under mmu_lock by rmap_write_protect before). Same reasoning applies to mark_page_dirty: the gfn has been marked as dirty via the pagefault path. The cost of hash table and memslot lookups are quite significant if the workload is pagetable write intensive resulting in increased mmu_lock contention. Signed-off-by: Marcelo Tosatti Signed-off-by: Avi Kivity --- arch/x86/kvm/mmu.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index fa3486d..dd20b19 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -1593,6 +1593,15 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *shadow_pte, spte |= PT_WRITABLE_MASK; + /* + * Optimization: for pte sync, if spte was writable the hash + * lookup is unnecessary (and expensive). Write protection + * is responsibility of mmu_get_page / kvm_sync_page. + * Same reasoning can be applied to dirty page accounting. + */ + if (!can_unsync && is_writeble_pte(*shadow_pte)) + goto set_pte; + if (mmu_need_write_protect(vcpu, gfn, can_unsync)) { pgprintk("%s: found shadow page for %lx, marking ro\n", __func__, gfn); -- 1.8.2.3