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Re: [Xen-devel] [PATCH 0/4] x86: Drop PVRDTSCP and fix MSR_TSC_AUX handling for guests


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 16 Nov 2018 17:43:42 +0000
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  • Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 16 Nov 2018 17:44:08 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 16/11/2018 10:11, Jan Beulich wrote:
>>>> On 15.11.18 at 22:47, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Boris has confirmed that noone appears to be using PVRDTSCP any more, and in
>> the decade since it was introduced, guest kernel / hardware support has
>> provided a better alternative.
> Doesn't removal of functionality require knowing that it was never used
> at all, rather than just knowing that nothing uses it anymore? What if
> some old guest somewhere relies on it?

Its an all-or-nothing feature.  The entirety of your VM userspace need
to support it, or timing will go wrong on migrate.

We already established that it appears to be a vestigial Oracle-ism for
which no consumer side code ever appeared, and that isn't used.

What is unacceptable is PVRDTSCP's implementation causing breakages in
architectural behaviour for non-PVRDTSCP configurations, and one way or
another, this needs fixing.

Please can we make a decision, because I don't have to time (or indeed,
the want) to and fix this a 3rd different way if that's going to run
into a similar reaction.

~Andrew

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