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

[Jan Kiszka <jan.kiszka@web.de>]

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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.
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.

Jan


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