Re: [PATCH] irqchip/gic-v3-its: Limit number of per-device MSIs to the range the ITS supports

[Robin Murphy <robin.murphy@arm.com>]

On 2026-02-06 3:48 pm, Marc Zyngier wrote:
> The ITS driver blindly assumes that EventIDs are in abundant supply,
> to the point where it never checks how many the HW actually supports.
> 
> It turns out that some pretty esoteric integrations make it so that
> only a few bits are available, all the way down to a. single. bit.
> 
> Enforce the advertised limitation at the point of allocating the
> device structure, and hope that the endpoint driver can deal with
> such limitation.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>   drivers/irqchip/irq-gic-v3-its.c   | 4 ++++
>   include/linux/irqchip/arm-gic-v3.h | 1 +
>   2 files changed, 5 insertions(+)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 2988def30972b..a51e8e6a81819 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -3475,6 +3475,7 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>   	int lpi_base;
>   	int nr_lpis;
>   	int nr_ites;
> +	int id_bits;
>   	int sz;
>   
>   	if (!its_alloc_device_table(its, dev_id))
> @@ -3486,7 +3487,10 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>   	/*
>   	 * Even if the device wants a single LPI, the ITT must be
>   	 * sized as a power of two (and you need at least one bit...).
> +	 * Also honor the ITS's own EID limit.
>   	 */
> +	id_bits = FIELD_GET(GITS_TYPER_IDBITS, its->typer) + 1;
> +	nvecs = min_t(unsigned int, nvecs, BIT(id_bits));
>   	nr_ites = max(2, nvecs);
>   	sz = nr_ites * (FIELD_GET(GITS_TYPER_ITT_ENTRY_SIZE, its->typer) + 1);
>   	sz = max(sz, ITS_ITT_ALIGN);
> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
> index 70c0948f978eb..0225121f30138 100644
> --- a/include/linux/irqchip/arm-gic-v3.h
> +++ b/include/linux/irqchip/arm-gic-v3.h
> @@ -394,6 +394,7 @@
>   #define GITS_TYPER_VLPIS		(1UL << 1)
>   #define GITS_TYPER_ITT_ENTRY_SIZE_SHIFT	4
>   #define GITS_TYPER_ITT_ENTRY_SIZE	GENMASK_ULL(7, 4)
> +#define GITS_TYPER_IDBITS		GENMASK_ULL(12, 8)
>   #define GITS_TYPER_IDBITS_SHIFT		8
>   #define GITS_TYPER_DEVBITS_SHIFT	13
>   #define GITS_TYPER_DEVBITS		GENMASK_ULL(17, 13)