Re: [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()

From: Tim Deegan <tim@xen.org>
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>
Subject: Re: [PATCH] x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()
Date: Tue, 11 Aug 2015 16:45:26 +0100	[thread overview]
Message-ID: <20150811154526.GC884@deinos.phlegethon.org> (raw)
In-Reply-To: <55CA32C90200007800099B76@prv-mh.provo.novell.com>

At 09:37 -0600 on 11 Aug (1439285833), Jan Beulich wrote:
> >>> On 11.08.15 at 16:34, <tim@xen.org> wrote:
> > 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.
> 
> And how does this help when the mapping survives until the guest
> gets suspended?

Suspended is fine, so long as it happens before the final read of the
bitmap.

> And why would doing it _again_ when unmapping
> be better than doing it _only_ then?

My mistake - it is of course done _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.

Yep!

Tim.