Date: Tue, 25 Jun 2024 15:54:28 +0200
From: Roman Kagan <rkagan@...zon.de>
To: Marc Zyngier <maz@...nel.org>
CC: <linux-arm-kernel@...ts.infradead.org>, Catalin Marinas
	<catalin.marinas@....com>, Will Deacon <will@...nel.org>,
	<nh-open-source@...zon.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] irqchip/gicv3-its: Workaround for GIC-700 erratum 2195890

On Tue, Jun 25, 2024 at 09:45:22AM +0100, Marc Zyngier wrote:
> On Mon, 24 Jun 2024 17:55:41 +0100,
> Roman Kagan <rkagan@...zon.de> wrote:
> >
> > According to Arm CoreLink GIC-700 erratum 2195890, on GIC revisions
> > r0p0, r0p1, r1p0 under certain conditions LPIs may remain in the Pending
> > Table until one of a number of external events occurs.
> 
> Please add a link to the errata document.

https://developer.arm.com/documentation/SDEN-1769194/
Will include when respinning.

> >
> > No LPIs are lost but they may not be delivered in a finite time.
> >
> > The workaround is to issue an INV using GICR_INVLPIR to an unused, in
> > range LPI ID to retrigger the search.
> >
> > Add this workaround to the quirk table.  When the quirk is applicable,
> > carve out one LPI ID from the available range and run periodic work to
> > do INV to it, in order to prevent GIC from stalling.
> 
> The errata document says a lot more:
> 
> <quote>
> For physical LPIs the workaround is to issue an INV using GICR_INVLPIR
> to an unused, in range LPI ID to retrigger the search. This could be
> done periodically, for example, in line with a residency change, or as
> part of servicing LPIs.  If using LPIs as the event, then the
> GICR_INVLPIR write could be issued after servicing every LPI.
> 
> However, it only needs to be issued if:
> 
> * At least 4 interrupts in the block of 32 are enabled and mapped to
>   the current PE or, if easier,
> 
> * At least 4 interrupts in the block of 32 are enabled and mapped to
>   any PE
> </quote>

It didn't feel like worth optimizing for.  I'll reconsider.

> > TT: https://t.corp.amazon.com/D82032616
> 
> Gniii????

Indeed Q-/

> > Signed-off-by: Elad Rosner <eladros@...zon.com>
> > Signed-off-by: Mohamed Mediouni <mediou@...zon.com>
> > Signed-off-by: Roman Kagan <rkagan@...zon.de>
> 
> Who is the author?

Joint effort aka inherited ownership.  Will fix according to the
process doc.

> > +static void __maybe_unused its_quirk_gic700_2195890_work_handler(struct work_struct *work)
> > +{
> > +     int cpu;
> > +     void __iomem *rdbase;
> > +     u64 gicr_invlpir_val;
> > +
> > +     for_each_online_cpu(cpu) {
> 
> The errata document doesn't say that this need to happen for *every*
> RD. Can you please clarify this?

(Digging out a year-old comms with ARM)
> > In multi-chip GIC system, does this write have to happen in each
> > chip or would a write to a single GICR trigger the search in all
> > GICDs?
> The write needs to occur for each physical PE - in other words, to
> each individual GICR that the search needs to be re-triggered for.

> > +             raw_spin_lock(&gic_data_rdist_cpu(cpu)->rd_lock);
> > +             gic_write_lpir(gicr_invlpir_val, rdbase + GICR_INVLPIR);
> > +             raw_spin_unlock(&gic_data_rdist_cpu(cpu)->rd_lock);
> 
> No synchronisation? How is that supposed to work?
> 
> Also, if you need to dig into the internals of the driver, extract a
> helper from __direct_lpi_inv().

ACK

> > +     }
> > +
> > +     schedule_delayed_work(&its_quirk_gic700_2195890_data.work,
> > +             msecs_to_jiffies(ITS_QUIRK_GIC700_2195890_PERIOD_MSEC));
> 
> It would be pretty easy to detect whether an LPI was ack'ed since the
> last pass, and not issue the invalidate.

Makes sense, will look into it.

Overall, do you think this approach with a global work looping over cpus
is the right one, or we should better try and implement something
per-cpu?

> > +}
> > +
> > +static bool __maybe_unused its_enable_quirk_gic700_2195890(void *data)
> > +{
> > +     struct its_node *its = data;
> > +
> > +     if (its_quirk_gic700_2195890_data.lpi)
> > +             return true;
> > +
> > +     /*
> > +      * Use one LPI INTID from the start of the LPI range for GIC prodding,
> > +      * and make it unavailable for regular LPI use later.
> > +      */
> > +     its_quirk_gic700_2195890_data.lpi = lpi_id_base++;
> > +
> > +     INIT_DELAYED_WORK(&its_quirk_gic700_2195890_data.work,
> > +                       its_quirk_gic700_2195890_work_handler);
> > +     schedule_delayed_work(&its_quirk_gic700_2195890_data.work, 0);
> > +
> > +     return true;
> > +}
> 
> It is a bit odd to hook this on an ITS being probed when the ITS isn't
> really involved. Not a big deal, but a bit clumsy.

True, but the LPI allocation lives in this file so it looked easier to
wire it all up here.  Where do you think it's more appropriate?

> >  static const struct gic_quirk its_quirks[] = {
> >  #ifdef CONFIG_CAVIUM_ERRATUM_22375
> >       {
> > @@ -4822,6 +4879,17 @@ static const struct gic_quirk its_quirks[] = {
> >               .property = "dma-noncoherent",
> >               .init   = its_set_non_coherent,
> >       },
> > +#ifdef CONFIG_ARM64_ERRATUM_2195890
> > +     {
> > +             .desc   = "ITS: GIC-700 erratum 2195890",
> > +             /*
> > +              * Applies to r0p0, r0p1, r1p0: iidr_var(bits 16..19) == 0 or 1
> > +              */
> > +             .iidr   = 0x0400043b,
> > +             .mask   = 0xfffeffff,
> > +             .init   = its_enable_quirk_gic700_2195890,
> 
> This catches r0p0 and r1p0, but not r0p1 (you require that bits 15:12
> are 0).

Ouch, right.  Given the erratum exact wording

> Fault Status: Present in: r0p0, r0p1, r1p0 Fixed in: r2p0

I guess I should match everything below r2p0 and allow arbitrary bits
15:12 (i.e. set the third nibble in the mask to 0).

> Overall, this requires a bit of rework. Notably, this could be
> significantly relaxed to match the requirements of the published
> workaround.

Thanks for the propmpt review!  Will rework and respin.
Roman.



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