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

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Message-ID: <8d17a56c-63b5-4d5c-836c-dfc150dcbfc8@linux.dev>
Date: Mon, 9 Feb 2026 02:48:41 +0800
From: Zenghui Yu <zenghui.yu@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 Thomas Gleixner <tglx@...nel.org>, Robin Murphy <robin.murphy@....com>,
 stable@...r.kernel.org
Subject: Re: [PATCH] irqchip/gic-v3-its: Limit number of per-device MSIs to
 the range the ITS supports

On 2/6/26 11:48 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.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Cc: stable@...r.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)

Reviewed-by: Zenghui Yu <zenghui.yu@...ux.dev>

Thanks,
Zenghui