Re: [Xen-devel] [PATCH 0 of 6] [RFC] Use wait queues for paging, v2

[Tim Deegan <tim@xxxxxxx>]

At 16:37 +0100 on 15 Mar (1331829432), Olaf Hering wrote:
> On Thu, Mar 15, Tim Deegan wrote:
> 
> > At 20:56 +0100 on 29 Feb (1330548983), Olaf Hering wrote:
> > > On Wed, Feb 29, Olaf Hering wrote:
> > > 
> > > > This is the domain_lock() in xenmem_add_to_physmap_once(). 
> > > > 
> > > > Is get_gfn_untyped() correct, or would get_gfn_query() work as well in
> > > > this context?
> > 
> > get_gfn_untyped() is correct.  I'm not sure that we really need to take
> > the domain lock while we're doing it, though.  It might be that the new
> > gfn locks will be enough to serialize these updates.
> > 
> > > Another case is emulate_privileged_op(), in "Write CR3" case
> > > get_gfn_untyped() is called with domain_lock().
> > 
> > I think that should be OK as it only happens for PV guests. 
> 
> I think its just me not knowing these things, but would it make sense to
> add comments to XX_lock users describing what they protect (or used to
> protect since comments can stale over time)?

Yes!  I'm trying to get this sorted out for all the MM locks, as well as
the discipline of which order to take them in.  But the domain lock
(a.k.a the Big Lock) is an ancient and venerable lock, now poorly
understood, at least by me. :)  The add_to_physmap path started taking
this lock in cset 9187:fbeb0a5b7219, in 2006.

Tim.


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