Re: [PATCH v4] VMX: use a single, global APIC access page

[Tim Deegan <tim@xen.org>]

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