Re: [PATCH v8 7/7] KVM: x86: Expose TSC offset controls to userspace

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/10/21 21:11, Marcelo Tosatti wrote:
> That said, the point is: why not advance the_TSC_  values
> (instead of kvmclock nanoseconds), as doing so would reduce
> the "the CLOCK_REALTIME delay which is introduced during migration"
> for both kvmclock users and modern tsc clocksource users.

It already does, that's the cool part.  Take again the formula here:

    guest_off_1 = t_0 + guest_off_0 + (k_1 - k_0) * freq - t_1

and set:

	t_1 = t_0 + host_off_0_1 + (k_1 - k_0) * freq

i.e. t_0 and t_1 are different because 1) the machines were booted at 
different times, which is host_off_0_1 2) t_1 includes the migration 
downtime between k_0 and k_1

Now you have:

    guest_off_1 = t_0 + guest_off_0 + (k_1 - k_0) * freq
	       - t_0 - real_off_n - (k_1 - k_0) * freq

    guest_off_1 = guest_off_0 - host_off_0_1

That is, the TSC is exactly the same as it was on the source, just 
adjusted because the two machines were booted at different times.

The need to have precise (ns, cycle) pairings is exactly because it 
ensures that everything cancels in the formula, and all that is left is 
the differences in the TSC of the two hosts.

Paolo

> So yes, i also like this patchset, but would like it even more
> if it fixed the case above as well (and not sure whether adding
> the migration delta to KVMCLOCK makes it harder to fix TSC case
> later).