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From: Tim Deegan <tim@xen.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Kevin Tian" <kevin.tian@intel.com>,
	"Jun Nakajima" <jun.nakajima@intel.com>
Subject: Re: [PATCH v4] VMX: use a single, global APIC access page
Date: Thu, 22 Apr 2021 08:42:20 +0100	[thread overview]
Message-ID: <YIEo3DQojgc0zlzw@deinos.phlegethon.org> (raw)
In-Reply-To: <d698a29e-fb4e-a650-9e48-ad209e64c800@suse.com>

At 13:25 +0200 on 19 Apr (1618838726), Jan Beulich wrote:
> On 17.04.2021 21:24, Tim Deegan wrote:
> > At 12:40 +0200 on 12 Apr (1618231248), Jan Beulich wrote:
> >> By making this page global, we also eliminate the need to refcount it,
> >> or to assign it to any domain in the first place.
> > 
> > What is the aim here?  To save 4k per domain?  It seems to come out
> > about even for adding and removing code. 
> 
> True, but still it looks wrong to me to use a page per guest when one
> her host suffices. Think about many tiny, short-lived VMs (as in
> Tamas'es VM forking).

OK, fair enough.

> >> --- a/xen/arch/x86/mm/shadow/set.c
> >> +++ b/xen/arch/x86/mm/shadow/set.c
> >> @@ -94,6 +94,22 @@ shadow_get_page_from_l1e(shadow_l1e_t sl
> >>      ASSERT(!sh_l1e_is_magic(sl1e));
> >>      ASSERT(shadow_mode_refcounts(d));
> >>  
> >> +    /*
> >> +     * VMX'es APIC access MFN is just a surrogate page.  It doesn't actually
> >> +     * get accessed, and hence there's no need to refcount it (and refcounting
> >> +     * would fail, due to the page having no owner).
> >> +     */
> >> +    if ( mfn_valid(mfn = shadow_l1e_get_mfn(sl1e)) )
> > 
> > Would it be better to check specifically for mfn == apic_access_mfn
> > (and apic_access_mfn != 0, I guess)?
> 
> Roger did ask about the same - I neither want to expose apic_access_mfn
> outside its CU, nor do I want to introduce an accessor function. Both
> feel like layering violations to me.

I think that this is even more of a layering violation: what we
actually want is to allow un-refcounted mappings of the
apic_access_mfn, but to do it we're relying on an internal
implementation detail (that it happens to be un-owned and PGC_extra)
rather than giving ourselves an API.

And so we're tangled up talking about how to write comments to warn
our future selves about the possible side-effects.

> >  If we want this behaviour for
> > for all un-owned PGC_extra MFNs it would be good to explain that in the
> > comments.
> 
> This is hard to tell without knowing which (or even if) further such
> PGC_extra pages will appear. Hence any comment to that effect would be
> guesswork at best. Of course I can add e.g. "Other pages with the same
> properties would be treated the same", if that's what you're after?

If you want to go this way there should be a comment here saying that
we're allowing this for all PGC_extra pages because we need it for
apic_access_mfn, and a comment at PGC_extra saying that it has this
effect.

Cheers,

Tim


  reply	other threads:[~2021-04-22  7:42 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-22 10:55 [PATCH v3 0/2] VMX: apic access page handling adjustments Jan Beulich
2021-02-22 10:56 ` [PATCH v3 1/2][4.15] VMX: delay p2m insertion of APIC access page Jan Beulich
2021-02-22 11:25   ` Ian Jackson
2021-02-22 14:05     ` Jan Beulich
2021-02-22 17:17       ` Ian Jackson
2021-02-22 12:15   ` Roger Pau Monné
2021-02-25  8:44   ` Jan Beulich
2021-02-26  7:06     ` Tian, Kevin
2021-02-22 10:57 ` [PATCH v3 2/2] VMX: use a single, global " Jan Beulich
2021-03-01  2:34   ` Tian, Kevin
2021-03-01  8:18     ` Jan Beulich
2021-04-12 10:40 ` [PATCH v4] " Jan Beulich
2021-04-12 15:31   ` Roger Pau Monné
2021-04-13  9:24     ` Jan Beulich
2021-04-13 10:18       ` Roger Pau Monné
2021-04-13 12:03         ` Jan Beulich
2021-04-13 13:03           ` Roger Pau Monné
2021-04-17 19:24   ` Tim Deegan
2021-04-19 11:25     ` Jan Beulich
2021-04-22  7:42       ` Tim Deegan [this message]
2021-04-22  9:38         ` Jan Beulich
2021-04-22 15:05           ` Tim Deegan
2021-04-23 10:51 ` [PATCH v4 0/3] VMX APIC access page and shadow adjustments Jan Beulich
2021-04-23 10:52   ` [PATCH v4 1/3] VMX: use a single, global APIC access page Jan Beulich
2021-04-23 14:17     ` Roger Pau Monné
2021-04-23 14:42       ` Jan Beulich
2021-04-26 17:55         ` Tim Deegan
2021-04-25  1:27     ` Tian, Kevin
2021-04-26 17:53     ` Tim Deegan
2021-04-23 10:53   ` [PATCH v4 2/3] x86/shadow: re-use variables in shadow_get_page_from_l1e() Jan Beulich
2021-04-23 10:54   ` [PATCH v4 3/3] x86/shadow: streamline shadow_get_page_from_l1e() Jan Beulich
2021-04-23 11:00   ` Really v5 (was: [PATCH v4 0/3] VMX APIC access page and shadow adjustments) Jan Beulich

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