Re: [PATCH v3 03/33] iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()

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

On 28/04/2025 9:54 pm, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> In situations where mapping/unmapping squence can be controlled by
> userspace, attempting to map over a region that has not yet been
> unmapped is an error.  But not something that should spam dmesg.
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>   drivers/iommu/io-pgtable-arm.c | 18 ++++++++++++------
>   include/linux/io-pgtable.h     |  8 ++++++++
>   2 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index f27965caf6a1..99523505dac5 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -475,7 +475,7 @@ static int __arm_lpae_map(struct arm_lpae_io_pgtable *data, unsigned long iova,
>   		cptep = iopte_deref(pte, data);
>   	} else if (pte) {
>   		/* We require an unmap first */
> -		WARN_ON(!selftest_running);
> +		WARN_ON(!selftest_running && !(cfg->quirks & IO_PGTABLE_QUIRK_NO_WARN_ON));

If we are going to have this as a general mechanism then the selftests 
should use it as well.

Thanks,
Robin.

>   		return -EEXIST;
>   	}
>   
> @@ -649,8 +649,10 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
>   	unmap_idx_start = ARM_LPAE_LVL_IDX(iova, lvl, data);
>   	ptep += unmap_idx_start;
>   	pte = READ_ONCE(*ptep);
> -	if (WARN_ON(!pte))
> -		return 0;
> +	if (!pte) {
> +		WARN_ON(!(data->iop.cfg.quirks & IO_PGTABLE_QUIRK_NO_WARN_ON));
> +		return -ENOENT;
> +	}
>   
>   	/* If the size matches this level, we're in the right place */
>   	if (size == ARM_LPAE_BLOCK_SIZE(lvl, data)) {
> @@ -660,8 +662,10 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
>   		/* Find and handle non-leaf entries */
>   		for (i = 0; i < num_entries; i++) {
>   			pte = READ_ONCE(ptep[i]);
> -			if (WARN_ON(!pte))
> +			if (!pte) {
> +				WARN_ON(!(data->iop.cfg.quirks & IO_PGTABLE_QUIRK_NO_WARN_ON));
>   				break;
> +			}
>   
>   			if (!iopte_leaf(pte, lvl, iop->fmt)) {
>   				__arm_lpae_clear_pte(&ptep[i], &iop->cfg, 1);
> @@ -976,7 +980,8 @@ arm_64_lpae_alloc_pgtable_s1(struct io_pgtable_cfg *cfg, void *cookie)
>   	if (cfg->quirks & ~(IO_PGTABLE_QUIRK_ARM_NS |
>   			    IO_PGTABLE_QUIRK_ARM_TTBR1 |
>   			    IO_PGTABLE_QUIRK_ARM_OUTER_WBWA |
> -			    IO_PGTABLE_QUIRK_ARM_HD))
> +			    IO_PGTABLE_QUIRK_ARM_HD |
> +			    IO_PGTABLE_QUIRK_NO_WARN_ON))
>   		return NULL;
>   
>   	data = arm_lpae_alloc_pgtable(cfg);
> @@ -1079,7 +1084,8 @@ arm_64_lpae_alloc_pgtable_s2(struct io_pgtable_cfg *cfg, void *cookie)
>   	struct arm_lpae_io_pgtable *data;
>   	typeof(&cfg->arm_lpae_s2_cfg.vtcr) vtcr = &cfg->arm_lpae_s2_cfg.vtcr;
>   
> -	if (cfg->quirks & ~(IO_PGTABLE_QUIRK_ARM_S2FWB))
> +	if (cfg->quirks & ~(IO_PGTABLE_QUIRK_ARM_S2FWB |
> +			    IO_PGTABLE_QUIRK_NO_WARN_ON))
>   		return NULL;
>   
>   	data = arm_lpae_alloc_pgtable(cfg);
> diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> index bba2a51c87d2..639b8f4fb87d 100644
> --- a/include/linux/io-pgtable.h
> +++ b/include/linux/io-pgtable.h
> @@ -88,6 +88,13 @@ struct io_pgtable_cfg {
>   	 *
>   	 * IO_PGTABLE_QUIRK_ARM_HD: Enables dirty tracking in stage 1 pagetable.
>   	 * IO_PGTABLE_QUIRK_ARM_S2FWB: Use the FWB format for the MemAttrs bits
> +	 *
> +	 * IO_PGTABLE_QUIRK_NO_WARN_ON: Do not WARN_ON() on conflicting
> +	 *	mappings, but silently return -EEXISTS.  Normally an attempt
> +	 *	to map over an existing mapping would indicate some sort of
> +	 *	kernel bug, which would justify the WARN_ON().  But for GPU
> +	 *	drivers, this could be under control of userspace.  Which
> +	 *	deserves an error return, but not to spam dmesg.
>   	 */
>   	#define IO_PGTABLE_QUIRK_ARM_NS			BIT(0)
>   	#define IO_PGTABLE_QUIRK_NO_PERMS		BIT(1)
> @@ -97,6 +104,7 @@ struct io_pgtable_cfg {
>   	#define IO_PGTABLE_QUIRK_ARM_OUTER_WBWA		BIT(6)
>   	#define IO_PGTABLE_QUIRK_ARM_HD			BIT(7)
>   	#define IO_PGTABLE_QUIRK_ARM_S2FWB		BIT(8)
> +	#define IO_PGTABLE_QUIRK_NO_WARN_ON		BIT(9)
>   	unsigned long			quirks;
>   	unsigned long			pgsize_bitmap;
>   	unsigned int			ias;