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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.02.2021 18:05, Ian Jackson wrote:
> Jan Beulich writes ("Re: [PATCH v2] VMX: use a single, global APIC access 
> page"):
>> While this patch was triggered not just by Julien's observation of
>> the early p2m insertion being a problem, but also many earlier
>> times of running into this odd code, it is - especially at this
>> stage - perhaps a possible option to split the change into just
>> the movement of the set_mmio_p2m_entry() invocation and all the
>> rest, in order to defer that rest until after 4.15.
> 
> I infer that this contains a bugfix, but perhaps other
> changes/improvements too.
> 
> George, I think you're our expert on this refcounting stuff - what do
> you think of this ?
> 
> I guess my key question is whether this change will introduce risk by
> messing with the complex refcounting machineryt - or remove it by
> removing an interaction with the refcounting.

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 ...

Jan