On Tue, 2024-06-25 at 23:34 +0200, Thomas Gleixner wrote:

On Tue, Jun 25 2024 at 20:01, David Woodhouse wrote:

quoted

From: David Woodhouse <redacted>

The vmclock "device" provides a shared memory region with precision clock
information. By using shared memory, it is safe across Live Migration.

Like the KVM PTP clock, this can convert TSC-based cross timestamps into
KVM clock values. Unlike the KVM PTP clock, it does so only when such is
actually helpful.

The memory region of the device is also exposed to userspace so it can be
read or memory mapped by application which need reliable notification of
clock disruptions.

There is effort underway to expose PTP clocks to user space via VDSO.

Ooh, interesting. Got a reference to that please?

 Can we please not expose an ad hoc interface for that?

Absolutely. I'm explicitly trying to intercept the virtio-rtc
specification here, to *avoid* having to do anything ad hoc.

Note that this is a "vDSO-style" interface from hypervisor to guest via
a shared memory region, not necessarily an actual vDSO.

But yes, it *is* intended to be exposed to userspace, so that userspace
can know the *accurate* time without a system call, and know that it
hasn't been perturbed by live migration.

As you might have heard the sad news, I'm not feeling up to the task to
dig deeper into this right now. Give me a couple of days to look at this
with working brain.

I have not heard any news, although now I'm making inferences.

Wishing you the best!