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

Jan Kiszka
2010-02-25T08:49:38Z

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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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