Re: [Qemu-devel] [PATCH] target-i386: clear guest TSC on reset

[Marcelo Tosatti]

On Fri, Dec 06, 2013 at 12:42:44AM +0900, Fernando Luis Vazquez Cao wrote:
> (2013/12/05 22:53), Paolo Bonzini wrote:
> > Il 05/12/2013 14:15, Fernando Luis Vazquez Cao ha scritto:
> >>          /*
> >>           * KVM is yet unable to synchronize TSC values of multiple VCPUs 
> >> on
> >>           * writeback. Until this is fixed, we only write the offset to SMP
> >>           * guests after migration, desynchronizing the VCPUs, but avoiding
> >>           * huge jump-backs that would occur without any writeback at all.
> >>           */
> >> -        if (smp_cpus == 1 || env->tsc != 0) {
> >> +        if (smp_cpus == 1 || env->tsc != 0 || level == 
> >> KVM_PUT_RESET_STATE) {
> >>              kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
> >>          }
> > This is still a bit ugly, and desynchronizes the VCPUs on reset.
> 
> I agree it is a bit ugly, but in my testing QEMU seemed to loop over all
> the VCPUS fast enough for the kernel side kvm_write_tsc() to do a
> reasonable job of matching the offsets (the Linux guest did not mark
> the TSC unstable due to the TSCs being unsynchronized). Am I missing
> something?

Right, modern kernels (see kvm_write_tsc) perform synchronization, so in
theory the "KVM is yet unable to synchronize ..." code is not necessary
anymore.

I vote for dropping the thing entirely.

> > The main point of my outlined solution is that you only have one value
> > that is tracked, not one per VCPU (which in the case of migration adds
> > unpredictable latencies---for example due to emptying the migration
> > buffers).  We already save that value; all that's left is to use it
> > instead of env->tsc.
> 
> I understand the benefits of what you are proposing but, since it is
> wider is scope and it would be more difficult to backport, I would
> prefer to implement it as a follow-up patch, unless you think that
> the current patch as a standalone fix does more harm than good.
> 
> - Fernando