[Qemu-devel] Re: [PATCH v3 07/10] qemu-kvm: Cleanup/fix TSC and PV clock writeback

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Feb 25, 2010 at 12:58:26AM +0100, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > On Thu, Feb 25, 2010 at 12:45:55AM +0100, Jan Kiszka wrote:
> >> Marcelo Tosatti wrote:
> >>> On Wed, Feb 24, 2010 at 03:17:55PM +0100, Jan Kiszka wrote:
> >>>> Drop kvm_load_tsc in favor of level-dependent writeback in
> >>>> kvm_arch_load_regs. KVM's PV clock MSRs fall in the same category and
> >>>> should therefore only be written back on full sync.
> >>>>
> >>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>> ---
> >>>>  qemu-kvm-x86.c        |   19 +++++--------------
> >>>>  qemu-kvm.h            |    4 ----
> >>>>  target-i386/machine.c |    5 -----
> >>>>  3 files changed, 5 insertions(+), 23 deletions(-)
> >>>>
> >>>> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
> >>>> index 840c1c9..84fd7fa 100644
> >>>> --- a/qemu-kvm-x86.c
> >>>> +++ b/qemu-kvm-x86.c
> >>>> @@ -965,8 +965,11 @@ void kvm_arch_load_regs(CPUState *env, int level)
> >>>>          set_msr_entry(&msrs[n++], MSR_LSTAR  ,           env->lstar);
> >>>>      }
> >>>>  #endif
> >>>> -    set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME,  env->system_time_msr);
> >>>> -    set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK,  env->wall_clock_msr);
> >>>> +    if (level == KVM_PUT_FULL_STATE) {
> >>>> +        set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
> >>>> +        set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr);
> >>>> +        set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
> >>>> +    }
> >>> As things stand today, the TSC should only be written on migration. See
> >>> 53f658b3c33616a4997ee254311b335e59063289 in the kernel.
> >> Migration and power-up - that's what this patch ensures (=>
> >> KVM_PUT_FULL_STATE). Or where do you see any problem?
> >>
> >> Jan
> >>
> > 
> > The problem is it should not write on power up (the kernel attempts
> > to synchronize the TSCs in that case, see the commit).
> > 
> 
> OK, need to read this more carefully.
> 
> I do not yet understand the difference from user space POV: it tries to
> transfer the identical TSC values to all currently stopped VCPU threads.

guest tsc = host tsc + offset

So at the time you set_msr(TSC), the guest visible TSC starts ticking. 
For SMP guests, this does not happen exactly at the same time for all
vcpus.

> That should not be different if we are booting a fresh VM or loading a
> complete state of a migrated image. If it does, it looks like a KVM
> kernel deficit on first glance.

Yes it is a deficit. After migration TSCs of SMP guests go out of sync.
Zachary is working on that.

> 
> Jan
>