From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: shadow prefetch Date: Wed, 02 Jan 2008 11:51:16 +0200 Message-ID: <477B5E94.30301@qumranet.com> References: <10EA09EFD8728347A513008B6B0DA77A02976A8E@pdsmsx411.ccr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org To: "Dong, Eddie" Return-path: In-Reply-To: <10EA09EFD8728347A513008B6B0DA77A02976A8E-wq7ZOvIWXbNpB2pF5aRoyrfspsVTdybXVpNB7YpNyf8@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Dong, Eddie wrote: > Current shadow code do prefetch in FNAME(prefetch_page), but it is only > used > to choose shadow_notrap_nonpresent_pte or shadow_trap_nonpresent_pte. > > At least for L1 shadow, prefetching to get exact shadow L1 pte won't > cause > performance regression (though handling time increases slightly ) since > it > won't generate a new write protected page. > > I'm not sure that "slightly" is accurate, we need to fetch 512 or 1024 entries. On the other hand, we have the benefit of batching (a call to get_user_pages() with n = 512 is much faster than 512 calls. I think that this will improve fork()s, but reduce performance with sparsely-accessed memory maps, or with fork() followed by exec() immediately. Only benchmarking can tell if it is an overall win. Xen limits the prefetch to a subset of the pages, maybe that's a better approach. > for those L2+ shadow page tables, we can do similar for those pointed > gfn which > is already shadowed I think L2s are very rare compared to L1 page tables, so any benefit would be minor. -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/