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

  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: George Dunlap <George.Dunlap@xxxxxxxxxx>
  • Date: Mon, 22 Feb 2021 10:26:19 +0000
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  • Cc: Ian Jackson <iwj@xxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, "Julien Grall" <julien@xxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
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  • Thread-topic: [PATCH for-4.15 v2] VMX: use a single, global APIC access page

> On Feb 22, 2021, at 7:51 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> 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

Does it actually contain a bugfix?  It’s not at all clear to me from reading 
the description that it’s anything other than a clean-up.  If there’s something 
else that needs to be called out explicitly.

It should indeed theoretically make things safer long-term; the current 
vlapic_page allocation is using special-case of the refcounting rules, making 
it much more prone to being the subject of an “oversight”.  But at this point 
in the release we don’t have much time at all to shake out any potential bugs 
in the new implementation; as such I’d consider anything other than the minimum 
necessary to fix a bug to be not worth it.




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