Re: What's kvmclock's custom sched_clock for?

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Jan 07, 2016 at 12:41:34AM -0800, Andy Lutomirski wrote:
> On Wed, Jan 6, 2016 at 11:18 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > AFAICT KVM reliably passes a monotonic TSC through to guests, even if
> > the host suspends.  That's all that sched_clock needs, I think.
> >
> > So why does kvmclock have a custom sched_clock?
> >
> > On a related note, KVM doesn't pass the "invariant TSC" feature
> > through to guests on my machine even though "invtsc" is set in QEMU
> > and the kernel host code appears to support it.  What gives?
> 
> I think I solved part of the puzzle.  KVM doesn't like to advertise
> invtsc by default because that breaks migration.  (Oddly, the end
> result seems wrong -- with migration, the TSC doesn't stop, but it's
> not constant, and X86_FEATURE_CONSTANT_TSC is nonetheless set, but
> whatever.)  So the scheduler clock doesn't get marked stable.

Can you break down this sentence?

QEMU commit 68bfd0ad4a1dcc4c328d5db85dc746b49c1ec07e

    target-i386: block migration and savevm if invariant tsc is exposed
    
    Invariant TSC documentation mentions that "invariant TSC will run at a
    constant rate in all ACPI P-, C-. and T-states".
    
    This is not the case if migration to a host with different TSC
frequency
    is allowed, or if savevm is performed. So block migration/savevm.


> Is that it?
> 
> This still doesn't explain why even explicitly trying to set invtsc
> doesn't seem to work.
> 
> --Andy