linux-kernel - Re: [PATCH v11 1/6] irqchip/gic-v3: Enable support for SGIs to act as NMIs

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Date:   Sat, 26 Aug 2023 11:36:53 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kgdb-bugreport@...ts.sourceforge.net,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        linux-perf-users@...r.kernel.org,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Chen-Yu Tsai <wens@...e.org>,
        linux-arm-kernel@...ts.infradead.org, ito-yuichi@...itsu.com,
        Stephen Boyd <swboyd@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 1/6] irqchip/gic-v3: Enable support for SGIs to act as NMIs

On Thu, 24 Aug 2023 16:30:27 +0100,
Douglas Anderson <dianders@...omium.org> wrote:
> 
> As of commit 6abbd6988971 ("irqchip/gic, gic-v3: Make SGIs use
> handle_percpu_devid_irq()") SGIs are treated the same as PPIs/EPPIs
> and use handle_percpu_devid_irq() by default. Unfortunately,
> handle_percpu_devid_irq() isn't NMI safe, and so to run in an NMI
> context those should use handle_percpu_devid_fasteoi_nmi().
> 
> In order to accomplish this, we just have to make room for SGIs in the
> array of refcounts that keeps track of which interrupts are set as
> NMI. We also rename the array and create a new indexing scheme that
> accounts for SGIs.
> 
> Also, enable NMI support prior to gic_smp_init() as allocation of SGIs
> as IRQs/NMIs happen as part of this routine.
> 
> Co-developed-by: Sumit Garg <sumit.garg@...aro.org>
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> In v10 I removed the previous Reviewed-by and Tested-by tags since the
> patch contents changed pretty drastically.
> 
> I'll also note that this change is a little more black magic to me
> than others in this series. I don't have a massive amounts of
> familiarity with all the moving parts of gic-v3, so I mostly just
> followed Mark Rutland's advice [1]. Please pay extra attention to make
> sure I didn't do anything too terrible.
> 
> Mark's advice wasn't a full patch and I ended up doing a bit of work
> to translate it to reality, so I did not add him as "Co-developed-by"
> here. Mark: if you would like this tag then please provide it and your
> Signed-off-by. I certainly won't object.
> 
> [1] https://lore.kernel.org/r/ZNC-YRQopO0PaIIo@FVFF77S0Q05N.cambridge.arm.com
> 
> (no changes since v10)
> 
> Changes in v10:
> - Rewrite as needed for 5.11+ as per Mark Rutland and Sumit.
> 
>  drivers/irqchip/irq-gic-v3.c | 54 ++++++++++++++++++++++++------------
>  1 file changed, 36 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index eedfa8e9f077..49d18cf3f636 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -78,6 +78,8 @@ static DEFINE_STATIC_KEY_TRUE(supports_deactivate_key);
>  #define GIC_LINE_NR	min(GICD_TYPER_SPIS(gic_data.rdists.gicd_typer), 1020U)
>  #define GIC_ESPI_NR	GICD_TYPER_ESPIS(gic_data.rdists.gicd_typer)
>  
> +#define SGI_NR		16

Why 16? We only allocate 8, as the other 8 are potentially stolen by
the secure side. We do try and initialise them all so that they have a
known state if they were actually configured as Group-1NS, but we
don't use them.

I understand that this simplifies the indexing in the rdist_nmi_refs
array and I'm not going to cry over 32 wasted bytes, but this
definitely deserves a comment.

	M.

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