[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 12:45:55AM +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).