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

From: "Jan Beulich" <JBeulich@suse.com>
To: Tim Deegan <tim@xen.org>
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 09:37:13 -0600	[thread overview]
Message-ID: <55CA32C90200007800099B76@prv-mh.provo.novell.com> (raw)
In-Reply-To: <20150811143407.GB884@deinos.phlegethon.org>

>>> 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? 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