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

From: Tim Deegan <tim@xen.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Wei Liu <wei.liu2@citrix.com>,
	Jan Beulich <JBeulich@suse.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Eddie Dong <eddie.dong@intel.com>,
	Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>,
	suravee.suthikulpanit@amd.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: Mon, 27 Jul 2015 12:09:33 +0100	[thread overview]
Message-ID: <20150727110933.GA54789@deinos.phlegethon.org> (raw)
In-Reply-To: <55B22964.2030701@citrix.com>

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.

If we want to catch these bugs, it might be useful to have a flag
that the tools can set when stop-and-copy begins, to indicate any
subsequent mark_dirty() calls are "too late".

Cheers,

Tim.