On Tue, 2024-06-25 at 23:34 +0200, Thomas Gleixner wrote: > On Tue, Jun 25 2024 at 20:01, David Woodhouse wrote: > > From: David Woodhouse <dwmw@xxxxxxxxxxxx> > > > > 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!
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