Re: [PATCH v2 4/4] irqchip/gicv3: Add workaround for FUJITSU-MONAKA erratum E#030003

[Marc Zyngier <maz@kernel.org>]

On Fri, 16 Oct 2026 11:00:48 +0100,
Tomohiro Misono <misono.tomohiro@fujitsu.com> wrote:
> 
> From: Kohei Enju <enju.kohei@fujitsu.com>
> 
> On affected FUJITSU-MONAKA CPUs, the erratum may occur when an SGI is
> generated by writing to ICC_SGI0R_EL1, ICC_SGI1R_EL1, or ICC_ASGI1R_EL1,
> if the operation races with CPU interface processing triggered by the
> arrival of a higher-priority interrupt, an update to a pending
> interrupt, or a transition of the PE to the Sleep state. When the
> erratum occurs, the SGI is lost and the system register write does not
> complete, causing the issuing core to hang.
> 
> Guest accesses to these registers are not affected by this erratum, as
> they are trapped to EL2 and handled without issuing the affected system
> register write.
> 
> Work around the erratum by writing 1 to the bit corresponding to the
> target SGI INTID in GICR_ISPENDR0 of each target PE's GIC Redistributor,
> instead of writing to the affected system registers.
> 
> Accessing a target PE's GICR_ISPENDR0 requires its Redistributor base to
> have been discovered. Affected platforms conform to SBBR, which requires
> PSCI for secondary CPU boot. They therefore do not use the ACPI Parking
> protocol, which sends an IPI before the secondary CPU has initialized
> its Redistributor. With PSCI, a secondary CPU discovers its
> Redistributor before becoming an IPI target.
> 
> Signed-off-by: Kohei Enju <enju.kohei@fujitsu.com>

You are posting a patch on behalf of someone else, you need to add
your own SoB:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v7.3-rc7#n448

> ---
> Changes in v2:
> - Clarify the erratum description and document that virtual accesses are
>   unaffected.
> - Remove the gic_send_sgi_via_rdist() helper.
> - Remove the write-count rationale.
> - Use the CPU MIDR instead of the SoC ID for filtering.
> 
> Note that I have left the dsb(st) after the GICR_ISPENDR0 writes
> unchanged, pending clarification of the required completion point. If
> the writes must be observed by a specific component before
> gic_ipi_send_mask() returns, I will use the appropriate mechanism to
> provide that guarantee.

See below.

[...]

> @@ -1386,6 +1391,17 @@ static void gic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
>  	 */
>  	dsb(ishst);
>  
> +	if (static_branch_unlikely(&gic_fujitsu_030003_erratum)) {
> +		for_each_cpu(cpu, mask) {
> +			void __iomem *base = gic_data_rdist_sgi_base_cpu(cpu);
> +
> +			writel_relaxed(BIT(d->hwirq), base + GICR_ISPENDR0);
> +		}
> +
> +		dsb(st);

What is this DSB achieving? As far as I can tell, not much. The memory
type is nGnRE, so DSB doesn't force writes to reach the end-point, and
only guarantee ordering that system register SGI generation doesn't
provide.

So either

(a) there is a functional requirement on this HW to have a DSB, and
    needs to be documented,

(b) or there is a need for end-point synchronisation, which needs to
    be enforced by a read back (knowing that the sysreg doesn't
    provide this guarantee), and this needs to be documented,

(c) or this is completely superfluous and can be dropped

Which case are we in?

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.