[PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace

[Marcelo Tosatti]

On Mon, Aug 23, 2021 at 01:56:30PM -0700, Oliver Upton wrote:
> Paolo,
> 
> On Sun, Aug 15, 2021 at 5:11 PM Oliver Upton <oupton at google.com> wrote:
> >
> > To date, VMM-directed TSC synchronization and migration has been a bit
> > messy. KVM has some baked-in heuristics around TSC writes to infer if
> > the VMM is attempting to synchronize. This is problematic, as it depends
> > on host userspace writing to the guest's TSC within 1 second of the last
> > write.
> >
> > A much cleaner approach to configuring the guest's views of the TSC is to
> > simply migrate the TSC offset for every vCPU. Offsets are idempotent,
> > and thus not subject to change depending on when the VMM actually
> > reads/writes values from/to KVM. The VMM can then read the TSC once with
> > KVM_GET_CLOCK to capture a (realtime, host_tsc) pair at the instant when
> > the guest is paused.
> >
> > Cc: David Matlack <dmatlack at google.com>
> > Cc: Sean Christopherson <seanjc at google.com>
> > Signed-off-by: Oliver Upton <oupton at google.com>
> 
> Could you please squash the following into this patch? We need to
> advertise KVM_CAP_VCPU_ATTRIBUTES to userspace. Otherwise, happy to
> resend.
> 
> Thanks,
> Oliver

Oliver,

Is there QEMU support for this, or are you using your own
userspace with this?