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

[Oliver Upton]

Marcelo,

On Thu, Aug 26, 2021 at 09:48:36AM -0300, Marcelo Tosatti wrote:
> 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?

Apologies for not getting back to you on your first mail. Sadly, I am
using our own userspace for this. That being said, adding support to
QEMU shouldn't be too challenging. I can take a stab at it if it makes
the series more amenable to upstream, with the giant disclaimer that I
haven't done work in QEMU before. Otherwise, happy to review someone
else's implementation.

--
Thanks,
Oliver