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

[Fernando Luis Vazquez Cao <fernando_b1@lab.ntt.co.jp>]

(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?


> 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