(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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html