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Commit c126d94f authored by Xiao Guangrong's avatar Xiao Guangrong Committed by Marcelo Tosatti
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KVM: MMU: lazily drop large spte 



Currently, kvm zaps the large spte if write-protected is needed, the later
read can fault on that spte. Actually, we can make the large spte readonly
instead of making them un-present, the page fault caused by read access can
be avoided

The idea is from Avi:
| As I mentioned before, write-protecting a large spte is a good idea,
| since it moves some work from protect-time to fault-time, so it reduces
| jitter.  This removes the need for the return value.

This version has fixed the issue reported in 6b73a960, the reason of that
issue is that fast_page_fault() directly sets the readonly large spte to
writable but only dirty the first page into the dirty-bitmap that means
other pages are missed. Fixed it by only the normal sptes (on the
PT_PAGE_TABLE_LEVEL level) can be fast fixed

Reviewed-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 92a476cb

arch/x86/kvm/mmu.c

+18 −16
Original line number Diff line number Diff line
@@ -1176,8 +1176,7 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep) 


/*

 * Write-protect on the specified @sptep, @pt_protect indicates whether

 * spte writ-protection is caused by protecting shadow page table.

 * @flush indicates whether tlb need be flushed.

 * spte write-protection is caused by protecting shadow page table.

 *

 * Note: write protection is difference between drity logging and spte

 * protection:
@@ -1186,10 +1185,9 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep) 
 * - for spte protection, the spte can be writable only after unsync-ing

 *   shadow page.

 *

 * Return true if the spte is dropped.

 * Return true if tlb need be flushed.

 */

static bool

spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)

static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool pt_protect)

{

	u64 spte = *sptep;
@@ -1199,17 +1197,11 @@ spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect) 


	rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);



	if (__drop_large_spte(kvm, sptep)) {

		*flush |= true;

		return true;

	}



	if (pt_protect)

		spte &= ~SPTE_MMU_WRITEABLE;

	spte = spte & ~PT_WRITABLE_MASK;



	*flush |= mmu_spte_update(sptep, spte);

	return false;

	return mmu_spte_update(sptep, spte);

}



static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
@@ -1221,11 +1213,8 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, 


	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {

		BUG_ON(!(*sptep & PT_PRESENT_MASK));

		if (spte_write_protect(kvm, sptep, &flush, pt_protect)) {

			sptep = rmap_get_first(*rmapp, &iter);

			continue;

		}



		flush |= spte_write_protect(kvm, sptep, pt_protect);

		sptep = rmap_get_next(&iter);

	}
@@ -2876,6 +2865,19 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level, 
	if (!spte_is_locklessly_modifiable(spte))

		goto exit;



	/*

	 * Do not fix write-permission on the large spte since we only dirty

	 * the first page into the dirty-bitmap in fast_pf_fix_direct_spte()

	 * that means other pages are missed if its slot is dirty-logged.

	 *

	 * Instead, we let the slow page fault path create a normal spte to

	 * fix the access.

	 *

	 * See the comments in kvm_arch_commit_memory_region().

	 */

	if (sp->role.level > PT_PAGE_TABLE_LEVEL)

		goto exit;



	/*

	 * Currently, fast page fault only works for direct mapping since

	 * the gfn is not stable for indirect shadow page.

arch/x86/kvm/x86.c

+6 −2
Original line number Diff line number Diff line
@@ -7315,8 +7315,12 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, 
		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);

	/*

	 * Write protect all pages for dirty logging.

	 * Existing largepage mappings are destroyed here and new ones will

	 * not be created until the end of the logging.

	 *

	 * All the sptes including the large sptes which point to this

	 * slot are set to readonly. We can not create any new large

	 * spte on this slot until the end of the logging.

	 *

	 * See the comments in fast_page_fault().

	 */

	if ((change != KVM_MR_DELETE) && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))

		kvm_mmu_slot_remove_write_access(kvm, mem->slot);