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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Nov 16, 2018 at 05:43:42PM +0000, Andrew Cooper wrote:
> 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?

We have verified that it was never used.
> 
> 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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