Commit 6316e1c8 authored Feb 03, 2010 by Rik van Riel Committed by Marcelo Tosatti Mar 01, 2010

KVM: VMX: emulate accessed bit for EPT



Currently KVM pretends that pages with EPT mappings never got
accessed.  This has some side effects in the VM, like swapping
out actively used guest pages and needlessly breaking up actively
used hugepages.

We can avoid those very costly side effects by emulating the
accessed bit for EPT PTEs, which should only be slightly costly
because pages pass through page_referenced infrequently.

TLB flushing is taken care of by kvm_mmu_notifier_clear_flush_young().

This seems to help prevent KVM guests from being swapped out when
they should not on my system.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

parent 8f0b1ab6

arch/x86/kvm/mmu.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -838,9 +838,15 @@ static int kvm_age_rmapp(struct kvm kvm, unsigned long rmapp,
		u64 *spte;
		int young = 0;

		/* always return old for EPT */
		/*
		* Emulate the accessed bit for EPT, by checking if this page has
		* an EPT mapping, and clearing it if it does. On the next access,
		* a new EPT mapping will be established.
		* This has some overhead, but not as much as the cost of swapping
		* out actively used pages or breaking up actively used hugepages.
		*/
		if (!shadow_accessed_mask)
		return 0;
		return kvm_unmap_rmapp(kvm, rmapp, data);

		spte = rmap_next(kvm, rmapp, NULL);
		while (spte) {

Admin message