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SubjectRe: [PATCH v2] KVM: PPC: Defer vtime accounting 'til after IRQ handling
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On 15/10/2021 04:23, Nicholas Piggin wrote:
> Excerpts from Laurent Vivier's message of October 13, 2021 7:30 pm:
>> On 13/10/2021 01:18, Michael Ellerman wrote:
>>> Laurent Vivier <lvivier@redhat.com> writes:
>>>> Commit 112665286d08 moved guest_exit() in the interrupt protected
>>>> area to avoid wrong context warning (or worse), but the tick counter
>>>> cannot be updated and the guest time is accounted to the system time.
>>>>
>>>> To fix the problem port to POWER the x86 fix
>>>> 160457140187 ("Defer vtime accounting 'til after IRQ handling"):
>>>>
>>>> "Defer the call to account guest time until after servicing any IRQ(s)
>>>> that happened in the guest or immediately after VM-Exit. Tick-based
>>>> accounting of vCPU time relies on PF_VCPU being set when the tick IRQ
>>>> handler runs, and IRQs are blocked throughout the main sequence of
>>>> vcpu_enter_guest(), including the call into vendor code to actually
>>>> enter and exit the guest."
>>>>
>>>> Fixes: 112665286d08 ("KVM: PPC: Book3S HV: Context tracking exit guest context before enabling irqs")
>>>> Cc: npiggin@gmail.com
>>>> Cc: <stable@vger.kernel.org> # 5.12
>>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>>> ---
>>>>
>>>> Notes:
>>>> v2: remove reference to commit 61bd0f66ff92
>>>> cc stable 5.12
>>>> add the same comment in the code as for x86
>>>>
>>>> arch/powerpc/kvm/book3s_hv.c | 24 ++++++++++++++++++++----
>>>> 1 file changed, 20 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
>>>> index 2acb1c96cfaf..a694d1a8f6ce 100644
>>>> --- a/arch/powerpc/kvm/book3s_hv.c
>>>> +++ b/arch/powerpc/kvm/book3s_hv.c
>>> ...
>>>> @@ -4506,13 +4514,21 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
>>>>
>>>> srcu_read_unlock(&kvm->srcu, srcu_idx);
>>>>
>>>> + context_tracking_guest_exit();
>>>> +
>>>> set_irq_happened(trap);
>>>>
>>>> kvmppc_set_host_core(pcpu);
>>>>
>>>> - guest_exit_irqoff();
>>>> -
>>>> local_irq_enable();
>>>> + /*
>>>> + * Wait until after servicing IRQs to account guest time so that any
>>>> + * ticks that occurred while running the guest are properly accounted
>>>> + * to the guest. Waiting until IRQs are enabled degrades the accuracy
>>>> + * of accounting via context tracking, but the loss of accuracy is
>>>> + * acceptable for all known use cases.
>>>> + */
>>>> + vtime_account_guest_exit();
>>>
>>> This pops a warning for me, running guest(s) on Power8:
>>>
>>> [ 270.745303][T16661] ------------[ cut here ]------------
>>> [ 270.745374][T16661] WARNING: CPU: 72 PID: 16661 at arch/powerpc/kernel/time.c:311 vtime_account_kernel+0xe0/0xf0
>>
>> Thank you, I missed that...
>>
>> My patch is wrong, I have to add vtime_account_guest_exit() before the local_irq_enable().
>
> I thought so because if we take an interrupt after exiting the guest that
> should be accounted to kernel not guest.
>
>>
>> arch/powerpc/kernel/time.c
>>
>> 305 static unsigned long vtime_delta(struct cpu_accounting_data *acct,
>> 306 unsigned long *stime_scaled,
>> 307 unsigned long *steal_time)
>> 308 {
>> 309 unsigned long now, stime;
>> 310
>> 311 WARN_ON_ONCE(!irqs_disabled());
>> ...
>>
>> But I don't understand how ticks can be accounted now if irqs are still disabled.
>>
>> Not sure it is as simple as expected...
>
> I don't know all the timer stuff too well. The
> !CONFIG_VIRT_CPU_ACCOUNTING case is relying on PF_VCPU to be set when
> the host timer interrupt runs irqtime_account_process_tick runs so it
> can accumulate that tick to the guest?
>
> That probably makes sense then, but it seems like we need that in a
> different place. Timer interrupts are not guaranteed to be the first one
> to occur when interrupts are enabled.
>
> Maybe a new tick_account_guest_exit() and move PF_VCPU clearing to that
> for tick based accounting. Call it after local_irq_enable and call the
> vtime accounting before it. Would that work?

Hi Nick,

I think I will not have the time to have a look to fix that?

Could you try?

Thanks,
Laurent

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