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

Marcelo Tosatti
2010-02-24T23:49:47Z

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).