[edk2-devel] [PATCH v2] ArmPkg/TimerDxe: Delay End Of Interrupt Signal

[Ashish Singhal via groups.io]

Marc,

In the document ARM062-1010708621-30 (AArch64 Programmer's Guides Generic Timer), towards the end of section 3.4 it says: "When writing an interrupt handler for the timers, it is important that software clears the interrupt before deactivating the interrupt in the GIC. Otherwise, the GIC will re-signal the same interrupt again."

My change was in accordance with this. We only clear the interrupt when we update the compare value but were signaling EOI before that going against the guidance in the document.

Thanks
Ashish
________________________________
From: Marc Zyngier <maz at kernel.org>
Sent: Tuesday, October 12, 2021 2:57 AM
To: Ard Biesheuvel <ardb at kernel.org>; Ashish Singhal <ashishsingha at nvidia.com>
Cc: edk2-devel-groups-io <devel at edk2.groups.io>; Leif Lindholm <leif at nuviainc.com>; Ard Biesheuvel <ardb+tianocore at kernel.org>
Subject: Re: [PATCH v2] ArmPkg/TimerDxe: Delay End Of Interrupt Signal

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On Mon, 11 Oct 2021 23:24:38 +0100,
Ard Biesheuvel <ardb at kernel.org> wrote:
>
> (+ Marc)
>
> On Mon, 11 Oct 2021 at 23:40, Ashish Singhal <ashishsingha at nvidia.com> wrote:
> >
> > In an interrupt handler for the timers, it is important that
> > software clears the interrupt before deactivating the interrupt
> > in the GIC. Otherwise the GIC will re-signal the same interrupt
> > again.
> >
> > Signed-off-by: Ashish Singhal <ashishsingha at nvidia.com>
>
> Please don't spam us with almost identical versions of the same patch
> without even documenting what the difference is.
>
>
> > ---
> >  ArmPkg/Drivers/TimerDxe/TimerDxe.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/ArmPkg/Drivers/TimerDxe/TimerDxe.c b/ArmPkg/Drivers/TimerDxe/TimerDxe.c
> > index 0370620fae..46e5bbf287 100644
> > --- a/ArmPkg/Drivers/TimerDxe/TimerDxe.c
> > +++ b/ArmPkg/Drivers/TimerDxe/TimerDxe.c
> > @@ -300,10 +300,6 @@ TimerInterruptHandler (
> >    //
> >    OriginalTPL = gBS->RaiseTPL (TPL_HIGH_LEVEL);
> >
> > -  // Signal end of interrupt early to help avoid losing subsequent ticks
> > -  // from long duration handlers
> > -  gInterrupt->EndOfInterrupt (gInterrupt, Source);
> > -
> >    // Check if the timer interrupt is active
> >    if ((ArmGenericTimerGetTimerCtrlReg () ) & ARM_ARCH_TIMER_ISTATUS) {
> >
> > @@ -335,6 +331,11 @@ TimerInterruptHandler (
> >      ArmInstructionSynchronizationBarrier ();
> >    }
> >
> > +  // In an interrupt handler for the timers, it is important that software clears the interrupt
> > +  // before deactivating the interrupt in the GIC. Otherwise the GIC will re-signal the same
> > +  // interrupt again.
> > +  gInterrupt->EndOfInterrupt (gInterrupt, Source);
> > +
> >    gBS->RestoreTPL (OriginalTPL);
> >  }
> >

This isn't a requirement if you haven't re-enabled interrupts in
PSTATE (and the TPL being raised seems to indicate that interrupts are
actively masked here).

The timer is a level interrupt, and lowering the level takes time. If
your GIC implementation is good, it will notice that the lowering of
the level quickly, before you can reach the point where you re-enable
interrupts. If it is slow (or badly emulated if this is actually done
in a hypervisor), you'll get a spurious interrupt.

In any case, moving the EOI around doesn't make things any better. You
are just moving the goal post, without additional guarantees that the
level has been retired.

As a consequence, I don't think this patch makes much sense.

        M.

--
Without deviation from the norm, progress is not possible.


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