From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v2 10/16] KVM: MMU: fask check whether page is writable Date: Tue, 17 Apr 2012 20:10:55 +0800 Message-ID: <4F8D5DCF.1000702@linux.vnet.ibm.com> References: <4F87FA69.5060106@linux.vnet.ibm.com> <4F87FC82.2050302@linux.vnet.ibm.com> <4F8AE64F.7080909@redhat.com> <4F8B9115.2030807@linux.vnet.ibm.com> <4F8BEE1B.8000704@redhat.com> <4F8BF265.7020405@linux.vnet.ibm.com> <4F8C06ED.6020805@redhat.com> <4F8CE9CF.6050506@linux.vnet.ibm.com> <4F8D1EB5.2000302@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM To: Avi Kivity Return-path: Received: from e23smtp01.au.ibm.com ([202.81.31.143]:35575 "EHLO e23smtp01.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755174Ab2DQMLC (ORCPT ); Tue, 17 Apr 2012 08:11:02 -0400 Received: from /spool/local by e23smtp01.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 17 Apr 2012 12:03:18 +1000 In-Reply-To: <4F8D1EB5.2000302@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: 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.