Commit 03787394 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

KVM: x86/mmu: Avoid unnecessary flush on eager page split 



The TLB flush before installing the newly-populated lower level
page table is unnecessary if the lower-level page table maps
the huge page identically.  KVM knows it is if it did not reuse
an existing shadow page table, tell drop_large_spte() to skip
the flush in that case.

Extracted from a patch by David Matlack.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent ada51a9d

arch/x86/kvm/mmu/mmu.c

+20 −12
Original line number Diff line number Diff line
@@ -1135,7 +1135,7 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) 
		rmap_remove(kvm, sptep);

}



static void drop_large_spte(struct kvm *kvm, u64 *sptep)

static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush)

{

	struct kvm_mmu_page *sp;
@@ -1143,6 +1143,8 @@ static void drop_large_spte(struct kvm *kvm, u64 *sptep) 
	WARN_ON(sp->role.level == PG_LEVEL_4K);



	drop_spte(kvm, sptep);



	if (flush)

		kvm_flush_remote_tlbs_with_address(kvm, sp->gfn,

			KVM_PAGES_PER_HPAGE(sp->role.level));

}
@@ -2283,7 +2285,7 @@ static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator) 


static void __link_shadow_page(struct kvm *kvm,

			       struct kvm_mmu_memory_cache *cache, u64 *sptep,

			       struct kvm_mmu_page *sp)

			       struct kvm_mmu_page *sp, bool flush)

{

	u64 spte;
@@ -2291,10 +2293,11 @@ static void __link_shadow_page(struct kvm *kvm, 


	/*

	 * If an SPTE is present already, it must be a leaf and therefore

	 * a large one.  Drop it and flush the TLB before installing sp.

	 * a large one.  Drop it, and flush the TLB if needed, before

	 * installing sp.

	 */

	if (is_shadow_present_pte(*sptep))

		drop_large_spte(kvm, sptep);

		drop_large_spte(kvm, sptep, flush);



	spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));
@@ -2309,7 +2312,7 @@ static void __link_shadow_page(struct kvm *kvm, 
static void link_shadow_page(struct kvm_vcpu *vcpu, u64 *sptep,

			     struct kvm_mmu_page *sp)

{

	__link_shadow_page(vcpu->kvm, &vcpu->arch.mmu_pte_list_desc_cache, sptep, sp);

	__link_shadow_page(vcpu->kvm, &vcpu->arch.mmu_pte_list_desc_cache, sptep, sp, true);

}



static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
@@ -6170,6 +6173,7 @@ static void shadow_mmu_split_huge_page(struct kvm *kvm, 
	struct kvm_mmu_memory_cache *cache = &kvm->arch.split_desc_cache;

	u64 huge_spte = READ_ONCE(*huge_sptep);

	struct kvm_mmu_page *sp;

	bool flush = false;

	u64 *sptep, spte;

	gfn_t gfn;

	int index;
@@ -6187,20 +6191,24 @@ static void shadow_mmu_split_huge_page(struct kvm *kvm, 
		 * gfn-to-pfn translation since the SP is direct, so no need to

		 * modify them.

		 *

		 * If a given SPTE points to a lower level page table, installing

		 * such SPTEs would effectively unmap a potion of the huge page.

		 * This is not an issue because __link_shadow_page() flushes the TLB

		 * when the passed sp replaces a large SPTE.

		 * However, if a given SPTE points to a lower level page table,

		 * that lower level page table may only be partially populated.

		 * Installing such SPTEs would effectively unmap a potion of the

		 * huge page. Unmapping guest memory always requires a TLB flush

		 * since a subsequent operation on the unmapped regions would

		 * fail to detect the need to flush.

		 */

		if (is_shadow_present_pte(*sptep))

		if (is_shadow_present_pte(*sptep)) {

			flush |= !is_last_spte(*sptep, sp->role.level);

			continue;

		}



		spte = make_huge_page_split_spte(kvm, huge_spte, sp->role, index);

		mmu_spte_set(sptep, spte);

		__rmap_add(kvm, cache, slot, sptep, gfn, sp->role.access);

	}



	__link_shadow_page(kvm, cache, huge_sptep, sp);

	__link_shadow_page(kvm, cache, huge_sptep, sp, flush);

}



static int shadow_mmu_try_split_huge_page(struct kvm *kvm,