From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chao Peng Subject: Re: [PATCH v15 01/11] multicall: add no preemption ability between two calls Date: Fri, 12 Sep 2014 10:55:43 +0800 Message-ID: <20140912025543.GI15872@pengc-linux> References: <540EDB9B.6080908@citrix.com> <540F05ED020000780003299E@mail.emea.novell.com> <540EF624.4020200@citrix.com> <540F19AF0200007800032AD1@mail.emea.novell.com> <20140910013232.GG15872@pengc-linux> <54101D29.8010606@citrix.com> <54103F01020000780003328F@mail.emea.novell.com> <541024AF.8050404@citrix.com> <541043330200007800033304@mail.emea.novell.com> <54103207.5080004@citrix.com> Reply-To: Chao Peng Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <54103207.5080004@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andrew Cooper Cc: keir@xen.org, Ian.Campbell@citrix.com, stefano.stabellini@eu.citrix.com, George.Dunlap@eu.citrix.com, Ian.Jackson@eu.citrix.com, xen-devel@lists.xen.org, Jan Beulich , dgdegra@tycho.nsa.gov List-Id: xen-devel@lists.xenproject.org On Wed, Sep 10, 2014 at 12:12:07PM +0100, Andrew Cooper wrote: > On 10/09/14 11:25, Jan Beulich wrote: > >>>> On 10.09.14 at 12:15, wrote: > >> On 10/09/14 11:07, Jan Beulich wrote: > >>>>>> On 10.09.14 at 11:43, wrote: > >>>> Actually, on further thought, using multicalls like this cannot possibly > >>>> be correct from a functional point of view. > >>>> > >>>> Even with the no preempt flag between a wrmsr/rdmsr hypercall pair, > >>>> there is no guarantee that accesses to remote cpus msrs won't interleave > >>>> with a different natural access, clobbering the results of the wrmsr. > >>>> > >>>> However this is solved, the wrmsr/rdmsr pair *must* be part of the same > >>>> synchronous thread of execution on the appropriate cpu. You can trust > >>>> that interrupts won't play with these msrs, but you absolutely can't > >>>> guarantee that IPI/wrmsr/IPI/rdmsr will work. > >>> Not sure I follow, particularly in the context of the white listing of > >>> MSRs permitted here (which ought to not include anything the > >>> hypervisor needs control over). > >> Consider two dom0 vcpus both using this new multicall mechanism to read > >> QoS information for different domains, which end up both targeting the > >> same remote cpu. They will both end up using IPI/wrmsr/IPI/rdmsr, which > >> may interleave and clobber the first wrmsr. > > But that situation doesn't result from the multicall use here - it would > > equally be the case for an inherently batchable hypercall. > > Indeed - I called out multicall because of the current implementation, > but I should have been more clear. > > > To deal with > > that we'd need a wrmsr-then-rdmsr operation, or move the entire > > execution of the batch onto the target CPU. Since the former would > > quickly become unwieldy for more complex operations, I think this > > gets us back to aiming at using continue_hypercall_on_cpu() here. > > Which gets us back to the problem that you cannot use > copy_{to,from}_guest() after continue_hypercall_on_cpu(), due to being > in the wrong context. > > > I think this requires a step back and rethink. I can't offhand think of > any combination of existing bits of infrastructure which will allow this > to work correctly, which means something new needs designing. > How about this: 1) Still do the batch in do_platform_op() but add a iteration field in the interface structure. 2) Still use on_selected_cpus() but group the adjacent resource_ops which have a same cpu and NO_PREEMPT set into one and do it as a whole in the new cpu context. Chao > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel