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@xxxxxxxxxx> 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@xxxxxxxxx
>>> Cc: <stable@xxxxxxxxxxxxxxx> # 5.12
>>> Signed-off-by: Laurent Vivier <lvivier@xxxxxxxxxx>
>>> ---
>>>
>>> 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?
Thanks,
Nick
