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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 05, 2013 at 05:02:02PM +0100, Paolo Bonzini wrote:
> Il 05/12/2013 16:42, Fernando Luis Vazquez Cao ha scritto:
> > (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?
> 
> No, probably not.
> 
> > 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.
> 
> It does some harm in that it introduces a case where KVM_PUT_RESET_STATE
> restores something, but KVM_PUT_FULL_STATE doesn't.
> 
> If it really usually works, there shouldn't be a need for this "if"
> statement at all.
> 
> Marcelo, what do you think?
> 
> Paolo

Its OK to drop it, provided the following is tested on SMP guests:

1. initialization.
2. reboot.
3. migration.

With both stable and unstable TSC hosts (use wrmsr tool to write TSC on
a given host CPU, to make it an unstable TSC host). (A=rdmsr ; sleep 1s;
wrmsr A).

To make sure the code is not securing against a kvm_write_tsc
cornercase.