(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
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