tree 1291405b107b4caa495454855baeeea5b9baa5e8
parent 7850ac5420803996e2960d15b924021f28e0dffc
author Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> 1321262690 +0900
committer Avi Kivity <avi@redhat.com> 1324977440 +0200

KVM: Optimize dirty logging by rmap_write_protect()

Currently, write protecting a slot needs to walk all the shadow pages
and checks ones which have a pte mapping a page in it.

The walk is overly heavy when dirty pages in that slot are not so many
and checking the shadow pages would result in unwanted cache pollution.

To mitigate this problem, we use rmap_write_protect() and check only
the sptes which can be reached from gfns marked in the dirty bitmap
when the number of dirty pages are less than that of shadow pages.

This criterion is reasonable in its meaning and worked well in our test:
write protection became some times faster than before when the ratio of
dirty pages are low and was not worse even when the ratio was near the
criterion.

Note that the locking for this write protection becomes fine grained.
The reason why this is safe is descripted in the comments.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>