On 10 April 2024 11:29:13 BST, Paul Durrant <xadimgnik@xxxxxxxxx> wrote: >On 10/04/2024 10:52, Jack Allister wrote: >> + * It's possible that this vCPU doesn't have a HVCLOCK configured >> + * but the other vCPUs may. If this is the case calculate based >> + * upon the time gathered in the seqcount but do not update the >> + * vCPU specific PVTI. If we have one, then use that. > >Given this is a per-vCPU ioctl, why not fail in the case the vCPU doesn't have HVCLOCK configured? Or is your intention that a GET/SET should always work if TSC is stable? It definitely needs to work for SET even when the vCPU hasn't been run yet (and doesn't have a hvclock in vcpu->arch.hv_clock). I think it should ideally work for GET too. I did try arguing that if the vCPU hasn't set up its pvclock then why would it care if it's inaccurate? But there's a pathological case of AMP where one vCPU is dedicated to an RTOS or something, and only the *other* vCPUs bring up their pvclock. This of course brings you to the question of why we have it as a per-vCPU ioctl at all? It only needs to be done *once* to get/set the KVM-wide clock And a function of *this* vCPU's TSC. And the point is that if we're in use_master_clock mode, that's consistent across *all* vCPUs. There would be a bunch of additional complexity in making it a VM ioctl though, especially around the question of what to do if userspace tries to restore it when there *aren't* any vCPUs yet. So we didn't do that.
