linux-kernel - Re: [PATCH] KVM: MMU: lazily drop large spte

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Message-ID: <50A20428.1030004@linux.vnet.ibm.com>
Date:	Tue, 13 Nov 2012 16:26:16 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: MMU: lazily drop large spte

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@...ux.vnet.ibm.com>
>> ---
>>  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

Download attachment "testcase.tar.bz2" of type "application/x-bzip" (8444 bytes)