From 9b51a63024bd759f97a12f50907b8af23b065b36 Mon Sep 17 00:00:00 2001 From: Kai Huang Date: Wed, 28 Jan 2015 10:54:25 +0800 Subject: [PATCH] KVM: MMU: Explicitly set D-bit for writable spte. This patch avoids unnecessary dirty GPA logging to PML buffer in EPT violation path by setting D-bit manually prior to the occurrence of the write from guest. We only set D-bit manually in set_spte, and leave fast_page_fault path unchanged, as fast_page_fault is very unlikely to happen in case of PML. For the hva <-> pa change case, the spte is updated to either read-only (host pte is read-only) or be dropped (host pte is writeable), and both cases will be handled by above changes, therefore no change is necessary. Signed-off-by: Kai Huang Reviewed-by: Xiao Guangrong Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index c438224cca34..b7192236dcba 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -2597,8 +2597,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, } } - if (pte_access & ACC_WRITE_MASK) + if (pte_access & ACC_WRITE_MASK) { mark_page_dirty(vcpu->kvm, gfn); + spte |= shadow_dirty_mask; + } set_pte: if (mmu_spte_update(sptep, spte)) @@ -2914,6 +2916,18 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, */ gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->spt); + /* + * Theoretically we could also set dirty bit (and flush TLB) here in + * order to eliminate unnecessary PML logging. See comments in + * set_spte. But fast_page_fault is very unlikely to happen with PML + * enabled, so we do not do this. This might result in the same GPA + * to be logged in PML buffer again when the write really happens, and + * eventually to be called by mark_page_dirty twice. But it's also no + * harm. This also avoids the TLB flush needed after setting dirty bit + * so non-PML cases won't be impacted. + * + * Compare with set_spte where instead shadow_dirty_mask is set. + */ if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte) mark_page_dirty(vcpu->kvm, gfn); -- 2.20.1