Re: [4.17?] Re: [PATCH] common: map_vcpu_info() wants to unshare the underlying page

[Julien Grall <julien@xxxxxxx>]

Hi Henry, Jan,

Sorry for the late reply.

On 26/10/2022 03:03, Henry Wang wrote:
> > -----Original Message-----
> > From: Jan Beulich <jbeulich@xxxxxxxx>
> > Sent: Wednesday, October 26, 2022 12:07 AM
> > To: Henry Wang <Henry.Wang@xxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>;
> > Julien Grall <julien@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
> > Wei Liu <wl@xxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Subject: [4.17?] Re: [PATCH] common: map_vcpu_info() wants to unshare
> > the underlying page
> >
> > On 25.10.2022 17:42, Roger Pau Monné wrote:
> > > On Tue, Oct 11, 2022 at 10:48:38AM +0200, Jan Beulich wrote:
> > > > Not passing P2M_UNSHARE to get_page_from_gfn() means there won't
> > even be
> > > > an attempt to unshare the referenced page, without any indication to the
> > > > caller (e.g. -EAGAIN). Note that guests have no direct control over
> > > > which of their pages are shared (or paged out), and hence they have no
> > > > way to make sure all on their own that the subsequent obtaining of a
> > > > writable type reference can actually succeed.
> > > >
> > > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > I didn't Cc you on the initial submission because mem-sharing isn't a
> > supported feature, but upon reconsideration I thought I'd at least ask
> > whether you would want to give this a release-ack. I don't really see
> > any risk associated with it.
> By looking at the patch itself, this change seems ok to me, so I think
> I will not block it, but I think Arm maintainers' approval might be needed
> because of the discussion in [1], so I added them for their information.
> If Arm maintainers do not object the change, you can have my release-ack.
The P2M query type is so far ignored on Arm as we neither support 
populate-on-demand nor memsharing.
I am assuming that if we ever introduce any those of features, we would 
follow the same behavior as x86. So:
Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall