Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page [and 1 more messages]

From: Ian Jackson <iwj@xenproject.org>
To: "Jan Beulich" <jbeulich@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Cc: George Dunlap <George.Dunlap@citrix.com>,
	"xen-devel\@lists.xenproject.org"
	<xen-devel@lists.xenproject.org>,
	Kevin Tian <kevin.tian@intel.com>,
	"Julien  Grall" <julien@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page [and 1 more messages]
Date: Mon, 22 Feb 2021 11:22:50 +0000	[thread overview]
Message-ID: <24627.37898.442930.913809@mariner.uk.xensource.com> (raw)
In-Reply-To: <YDOC2ACTf0bmryG1@Air-de-Roger>, <b5759150-e028-ac68-b8b5-8abcea02b5d9@suse.com>

Roger Pau Monné writes ("Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page"):
> On Mon, Feb 22, 2021 at 08:51:59AM +0100, Jan Beulich wrote:
> > If anything, then the latter, but largely neither afaict - there's no
> > change in this regard here at all as far as the guest could affect
> > behavior, due to the page getting inserted as p2m_mmio_direct, and
> > guest_remove_page() having
> > 
> >     if ( p2mt == p2m_mmio_direct )
> >     {
> >         rc = clear_mmio_p2m_entry(d, gmfn, mfn, PAGE_ORDER_4K);
> >         goto out_put_gfn;
> >     }
> > 
> > before any refcounting logic is reached. The removal of interaction
> > is because now the page doesn't get associated with a domain (and
> > hence doesn't become subject to refcounting) at all.
> > 
> > The risk of the change stems from going from using a per-domain
> > page to using a single, system-wide one, which indeed was the subject
> > of v1 discussion. In any event the consideration towards splitting
> > the change would cover either concern. Perhaps I should really do so
> > and submit as v3 ...
> 
> I agree it would be less risky to keep using a per-domain page, and
> switch to a global one after the release. From the discussion in v1 I
> don't think we where able to spot any specific issues apart from
> guests possibly being able to access shared data in this page from
> passthrough devices. I would at least feel more confortable with
> that approach given the point we are in the release process.

Thanks to Roger and Jan for these comments which were very helpful to
me.

Jan Beulich writes ("Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page"):
> Hmm, yes. The change wanted making anyway, for a long time imo. Hence
> when putting together the patch I forgot to call out that as a side
> effect it addresses a memory leak, as reported by Julien. With the
> splitting of the two changes that'll be necessarily mentioned. I'm
> about to submit v3.

Great, I see it now.

Thanks,
Ian.