From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw() Date: Tue, 11 Aug 2015 09:37:13 -0600 Message-ID: <55CA32C90200007800099B76@prv-mh.provo.novell.com> References: <55B224660200007800095083@prv-mh.provo.novell.com> <55B22964.2030701@citrix.com> <20150727110933.GA54789@deinos.phlegethon.org> <55CA1A190200007800099975@prv-mh.provo.novell.com> <20150811143407.GB884@deinos.phlegethon.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1ZPBcA-00038G-4W for xen-devel@lists.xenproject.org; Tue, 11 Aug 2015 15:37:22 +0000 In-Reply-To: <20150811143407.GB884@deinos.phlegethon.org> Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Tim Deegan Cc: Kevin Tian , Wei Liu , suravee.suthikulpanit@amd.com, Andrew Cooper , Eddie Dong , Aravind Gopalakrishnan , Jun Nakajima , xen-devel , Boris Ostrovsky , Keir Fraser List-Id: xen-devel@lists.xenproject.org >>> On 11.08.15 at 16:34, wrote: > At 07:51 -0600 on 11 Aug (1439279513), Jan Beulich wrote: >> >>> On 27.07.15 at 13:09, wrote: >> > At 13:02 +0100 on 24 Jul (1437742964), Andrew Cooper wrote: >> >> On 24/07/15 10:41, Jan Beulich wrote: >> >> > Beyond that log-dirty handling in _hvm_map_guest_frame() looks bogus >> >> > too: What if a XEN_DOMCTL_SHADOW_OP_* gets issued and acted upon >> >> > between the setting of the dirty flag and the actual write happening? >> >> > I.e. shouldn't the flag instead be set in hvm_unmap_guest_frame()? >> >> >> >> It does indeed. (Ideally the dirty bit should probably be held high for >> >> the duration that a mapping exists, but that is absolutely infeasible to >> >> do). >> > >> > IMO that would not be very useful -- a well-behaved toolstack will >> > have to make sure that relevant mappings are torn down before >> > stop-and-copy. Forcing the dirty bit high in the meantime just makes >> > every intermediate pass send a wasted copy of the page, without >> > actually closing the race window if the tools are buggy. >> >> Making sure such mappings got torn down in time doesn't help >> when the most recent write happened _after_ the most recent >> clearing of the dirty flag in a pass prior to stop-and-copy. > > This is why e.g. __gnttab_unmap_common sets the dirty bit again > as it unmaps. And how does this help when the mapping survives until the guest gets suspended? And why would doing it _again_ when unmapping be better than doing it _only_ then? But in any event I read this as agreement that moving (or in the worst case replicating) the hvm_map_guest_frame_rw() one into hvm_unmap_guest_frame() would be an appropriate thing to do. Jan