From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tim Deegan <tim@xen.org>
Subject: Re: [PATCH] x86/HVM: honor p2m_ram_ro in
 hvm_map_guest_frame_rw()
Date: Tue, 11 Aug 2015 15:34:07 +0100
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To: Jan Beulich <JBeulich@suse.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Wei Liu <wei.liu2@citrix.com>, suravee.suthikulpanit@amd.com, Andrew Cooper <andrew.cooper3@citrix.com>, Eddie Dong <eddie.dong@intel.com>, Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>, Jun Nakajima <jun.nakajima@intel.com>, xen-devel <xen-devel@lists.xenproject.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Keir Fraser <keir@xen.org>
List-Id: xen-devel@lists.xenproject.org

At 07:51 -0600 on 11 Aug (1439279513), Jan Beulich wrote:
> >>> On 27.07.15 at 13:09, <tim@xen.org> 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.

Cheers,

Tim.