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

[Tim Deegan <tim@xen.org>]

At 11:38 +0200 on 22 Apr (1619091522), Jan Beulich wrote:
> On 22.04.2021 09:42, Tim Deegan wrote:
> > 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:
> >>>> --- 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.
> 
> So (along with a comment to this effect) how about I make
> page_suppress_refcounting() and page_refcounting_suppressed() helpers?
> The former would set PGC_extra on the page and assert the page has no
> owner, while the latter would subsume the checks done here.

That sounds good to me.

> The only
> question then is what to do with the ASSERT(type == p2m_mmio_direct):
> That's still a property of the APIC access MFN which may or may not
> hold for future such pages. (It can't be part of the new helper anyway
> as "put" doesn't have the type available.)

I think we might drop that assertion, since the new mehanism would be
more general.

Cheers,

Tim.