KVM: MMU: Fix false flooding when a pte points to page table

The KVM MMU tries to detect when a speculative pte update is not actually
used by demand fault, by checking the accessed bit of the shadow pte.  If
the shadow pte has not been accessed, we deem that page table flooded and
remove the shadow page table, allowing further pte updates to proceed
without emulation.

However, if the pte itself points at a page table and only used for write
operations, the accessed bit will never be set since all access will happen
through the emulator.

This is exactly what happens with kscand on old (2.4.x) HIGHMEM kernels.
The kernel points a kmap_atomic() pte at a page table, and then
proceeds with read-modify-write operations to look at the dirty and accessed
bits.  We get a false flood trigger on the kmap ptes, which results in the
mmu spending all its time setting up and tearing down shadows.

Fix by setting the shadow accessed bit on emulated accesses.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 844f2a89afbc..c2d066e185f4 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -243,6 +243,7 @@ struct kvm_vcpu_arch {
 	gfn_t last_pt_write_gfn;
 	int   last_pt_write_count;
 	u64  *last_pte_updated;
+	gfn_t last_pte_gfn;
 
 	struct {
 		gfn_t gfn;	/* presumed gfn during guest pte update */