[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 12:58:26AM +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);
     }


Jan


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