linux-kernel - Re: [PATCH v2 10/16] KVM: MMU: fask check whether page is writable

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Message-ID: <4F8D5DCF.1000702@linux.vnet.ibm.com>
Date:	Tue, 17 Apr 2012 20:10:55 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 10/16] KVM: MMU: fask check whether page is writable

On 04/17/2012 03:41 PM, Avi Kivity wrote:

> On 04/17/2012 06:55 AM, Xiao Guangrong wrote:
>> On 04/16/2012 07:47 PM, Avi Kivity wrote:
>>
>>> On 04/16/2012 01:20 PM, Xiao Guangrong wrote:
>>>>>>
>>>>>> It is used to avoid the unnecessary overload 
>>>>>
>>>>> It's overloading me :(
>>>>>
>>>>
>>>>
>>>> Sorry.
>>>>
>>>
>>> The trick is to send those in separate patchset so the maintainer
>>> doesn't notice.
>>>
>>
>>
>> Thanks for your suggestion, i will pay more attention on it in the
>> further.
>>
>> For this patch, what did you mean of "those"? You mean the whole
>> rmap.PTE_LIST_WP_BIT (fast check for shadow page table write protection
>> and host write protection) or just about host_page_write_protect
>> (for KSM only)?
> 
> All of it.  Let's start with just modifying sptes concurrently and only
> later add reading bits from rmap concurrently, if it proves necessary.
> 


Okay, i agree.

>>
>> If we do not have rmap.PTE_LIST_WP_BIT, there may have regression on
>> shadow mmu.
>>
>> Hmm, do i need implement rmap.PTE_LIST_WP_BIT, then fast page fault?
> 
> Let's try to measure the effect without rmap.PTE_LIST_WP_BIT.  Usually
> PTE chains for page tables are short so the effect would be small.  Of
> course we can't tell about all guest.
> 


It is not about rmap's spte, it is about sp.sync write-protect, if the sp.sync
is written, the fast page fault path will be triggered even if no migration and
no framebuffer.

I have done a quick test for kernbench for 10 times and get the average value
without xwindow:

keep rmap.PTE_LIST_WP_BIT: 53.494
comment rmap.PTE_LIST_WP_BIT checking in page_fault_can_be_fast: 53.948

Anyway, for good review, let move fast page fault in first and discuss this in
the separate patchset later.

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