Hi Marcelo, On 11/13/2012 07:10 AM, Marcelo Tosatti wrote: > On Mon, Nov 05, 2012 at 05:59:26PM +0800, Xiao Guangrong wrote: >> Do not drop large spte until it can be insteaded by small pages so that >> the guest can happliy read memory through it >> >> 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. >> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> >> --- >> arch/x86/kvm/mmu.c | 34 +++++++++------------------------- >> 1 files changed, 9 insertions(+), 25 deletions(-) > > Its likely that other 4k pages are mapped read-write in the 2mb range > covered by a read-only 2mb map. Therefore its not entirely useful to > map read-only. > It needs a page fault to install a pte even if it is the read access. After the change, the page fault can be avoided. > Can you measure an improvement with this change? I have a test case to measure the read time which has been attached. It maps 4k pages at first (dirt-loggged), then switch to large sptes (stop dirt-logging), at the last, measure the read access time after write protect sptes. Before: 23314111 ns After: 11404197 ns
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