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

[Jan Kiszka <jan.kiszka@siemens.com>]

Marcelo Tosatti wrote:
> On Thu, Feb 25, 2010 at 09:48:47AM +0100, Jan Kiszka wrote:
>> Marcelo Tosatti wrote:
>>> 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.
>> Ouch.
>>
>>>> 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.
>>>
>> OK, so we need a workaround, ideally without reintroducing hooks. Is
>> this one acceptable?
>>
>>
>> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
>> index 84fd7fa..285c05a 100644
>> --- a/qemu-kvm-x86.c
>> +++ b/qemu-kvm-x86.c
>> @@ -966,7 +966,15 @@ void kvm_arch_load_regs(CPUState *env, int level)
>>      }
>>  #endif
>>      if (level == KVM_PUT_FULL_STATE) {
>> -        set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
>> +        /*
>> +         * 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) {
>> +            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);
>>      }
> 
> Well, migration of the SMP TSCs is not precise, but it is better than
> zeroing the TSCs on migration. So something like a new state (migration
> only), or a hack that mimicks that behaviour, is needed :(

It is not precise, but the above code shouldn't behave differently
compared to the existing one: tcp != 0 => we are loading values from
some VM that ran before, i.e. we are migrating.

If somehow possible, I do not want to introduce a new writeback level
for an x86-only kernel quirk.

Jan

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