Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

From: Thomas Gleixner
Date: Fri Feb 17 2017 - 12:55:20 EST

On Fri, 17 Feb 2017, Andy Lutomirski wrote:
> On Fri, Feb 17, 2017 at 2:14 AM, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> > Not only. As far as I understand (and I *think* K. Y. pointed this out)
> > when VM is migrating to another host TSC page clocksource is disabled for
> > extended period of time so we're better off reading from MSR than
> > looping here. With regards to VDSO this means reverting to doing normal
> > syscall.
> That's a crappy design. The guest really ought to be able to
> distinguish "busy, try again" from "bail and use MSR".
> Thomas, I can see valid reasons why the hypervisor might want to
> temporarily disable shared page-based timing, but I think it's silly
> that it's conflated with indicating "retry".

It's beyond silly if that's the case.

> But if this is indeed the ABI, we're stuck with it.

No argument.