From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: Essay on an important Xen decision (long) Date: Wed, 11 Jan 2006 10:41:05 -0600 Message-ID: <43C53521.7050705@us.ibm.com> References: <516F50407E01324991DD6D07B0531AD59030F8@cacexc12.americas.cpqcorp.net> <43C53164.2070308@us.ibm.com> <200601111622.31835.mark.williamson@cl.cam.ac.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <200601111622.31835.mark.williamson@cl.cam.ac.uk> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Mark Williamson Cc: "Magenheimer, Dan (HP Labs Fort Collins)" , xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org Mark Williamson wrote: >>I imagine that you would have to always have shadow paging enable but >>you could still do bulk updates ala writable page tables so the >>performance cost should be minimal I would think. >> >>Trying to understand the memory system in more details so any additional >>info is much appreciate :-) >> >> > >I don't see why that couldn't perform decently, although it'd have more >overhead than allowing the guest to manage its pagetables directly... I >*thought* this was intended to be supported at some point, but I'm not sure >if it's been needed yet. Others may have more concrete numbers for the >performance - I think writable PTs got benchmarked against shadowing at some >point. > > Just to be thorough, was the shadow paging code a "pure" shadow page table where ever PTE write trapped to the hypervisor or were bulk PMD updates sent to the hypervisor? I'm surprised there would be a measurable difference with shadow paging as it should only require a potential allocation (which could be fast pathed) and in the normal case, a couple extra reads/writes. I would think that cost would be overshadowed by the original cost of the context switch. Of course, I guess it wouldn't be that much of a shock to me that the overhead is at least measurable... Regards, Anthony Liguori >Cheers, >Mark > > > >>Thanks, >> >>Anthony Liguori >> >> >> >>>If ia64 does decide to back off from the P==M route then I suspect VP >>>is the way to go (which is I think how ia64 domU's currently work >>>anyway). >>> >>> -- Keir >>> >>> >>_______________________________________________ >>Xen-devel mailing list >>Xen-devel@lists.xensource.com >>http://lists.xensource.com/xen-devel >> >> > > >