Re: [PATCH v2] KVM: PPC: Defer vtime accounting 'til after IRQ handling

[Laurent Vivier]

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().

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...

Thanks,
Laurent