Re: [Xen-devel] [PATCH] x86/tsc: limit the usage of synchronization rendezvous

[Jürgen Groß <jgross@xxxxxxxx>]

On 23.10.19 16:17, Jan Beulich wrote:
> On 23.10.2019 15:56, Roger Pau Monne wrote:
> > If Xen detects the TSC is unreliable it will set a rendezvous helper
> > that tries to synchronize the different CPUs TSC value by propagating
> > the one from CPU#0 and writing it into the IA32_TSC MSR on each CPU.
> >
> > When the system has a single thread and there are no hotplugable CPUs
> > doing the above just results in reading the TSC from CPU#0 and writing
> > it into the IA32_TSC MSR of CPU#0, which is pointless, so limit the
> > usage of the synchronization rendezvous to systems that have more than
> > one CPU, even if those CPUs are yet to be hotplugged.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > I'm not sure whether this is suitable for 4.13, being a performance
> > improvement but not fixing a functional bug.
> Unless we know there's a lot of use of Xen in UP mode, I'd say
> rather not. _If_ there was a lot of such use, then I think we
> ought to do more work towards performance there (like re-
> introducing SMP alternatives patching).
I believe the main use case is pv-shim.

> Of course in the end the decision is to be taken by Jürgen,
> whom you didn't even Cc.
I'm inclined not to Release-ack it.


Juergen

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