[PATCH v6 00/21] KVM: Add idempotent controls for migrating system counter state

[Oliver Upton]

On Wed, Aug 11, 2021 at 6:05 AM Paolo Bonzini <pbonzini at redhat.com> wrote:
>
> On 04/08/21 10:57, Oliver Upton wrote:
> > KVM's current means of saving/restoring system counters is plagued with
> > temporal issues. At least on ARM64 and x86, we migrate the guest's
> > system counter by-value through the respective guest system register
> > values (cntvct_el0, ia32_tsc). Restoring system counters by-value is
> > brittle as the state is not idempotent: the host system counter is still
> > oscillating between the attempted save and restore. Furthermore, VMMs
> > may wish to transparently live migrate guest VMs, meaning that they
> > include the elapsed time due to live migration blackout in the guest
> > system counter view. The VMM thread could be preempted for any number of
> > reasons (scheduler, L0 hypervisor under nested) between the time that
> > it calculates the desired guest counter value and when KVM actually sets
> > this counter state.
> >
> > Despite the value-based interface that we present to userspace, KVM
> > actually has idempotent guest controls by way of system counter offsets.
> > We can avoid all of the issues associated with a value-based interface
> > by abstracting these offset controls in new ioctls. This series
> > introduces new vCPU device attributes to provide userspace access to the
> > vCPU's system counter offset.
> >
> > Patch 1 addresses a possible race in KVM_GET_CLOCK where
> > use_master_clock is read outside of the pvclock_gtod_sync_lock.
> >
> > Patch 2 adopts Paolo's suggestion, augmenting the KVM_{GET,SET}_CLOCK
> > ioctls to provide userspace with a (host_tsc, realtime) instant. This is
> > essential for a VMM to perform precise migration of the guest's system
> > counters.
> >
> > Patches 3-4 are some preparatory changes for exposing the TSC offset to
> > userspace. Patch 5 provides a vCPU attribute to provide userspace access
> > to the TSC offset.
> >
> > Patches 6-7 implement a test for the new additions to
> > KVM_{GET,SET}_CLOCK.
> >
> > Patch 8 fixes some assertions in the kvm device attribute helpers.
> >
> > Patches 9-10 implement at test for the tsc offset attribute introduced in
> > patch 5.
>
> The x86 parts look good, except that patch 3 is a bit redundant with my
> idea of altogether getting rid of the pvclock_gtod_sync_lock.  That said
> I agree that patches 1 and 2 (and extracting kvm_vm_ioctl_get_clock and
> kvm_vm_ioctl_set_clock) should be done before whatever locking changes
> have to be done.

Following up on patch 3.

> Time is ticking for 5.15 due to my vacation, I'll see if I have some
> time to look at it further next week.
>
> I agree that arm64 can be done separately from x86.

Marc, just a disclaimer:

I'm going to separate these two series, although there will still
exist dependencies in the selftests changes. Otherwise, kernel changes
are disjoint.

--
Thanks,
Oliver