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Re: [Xen-devel] [PATCH v2 3/9] x86/mm: honor opt_pcid also for 32-bit PV domains


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 17 Sep 2019 20:00:04 +0100
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  • Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Tue, 17 Sep 2019 19:00:28 +0000
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On 17/09/2019 07:14, Jan Beulich wrote:
> I can't see any technical or performance reason why we should treat
> 32-bit PV different from 64-bit PV in this regard.

Well, other than the fact this setting is only read for a 64bit guest...

The reason it isn't set for 32bit guests is that there is no scenario
where we use it.

For PV guests in general, we could consider making a similar
optimisation to Linux and having a LRU list of 4 or so PCIDs to speed up
process context switches, which would be applicable to 32bit guests as
well (so long as someone can reason about the TLB flushing behaviour of
the PV ABI), but this sounds like a large quantity of work.

~Andrew

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