Re: [PATCH v4 05/40] iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()

[Will Deacon <will@kernel.org>]

On Wed, May 14, 2025 at 10:53:19AM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> In situations where mapping/unmapping sequence 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.
> 
> Now that there is a quirk, we can also drop the selftest_running
> flag, and use the quirk instead for selftests.
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> Acked-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>  drivers/iommu/io-pgtable-arm.c | 27 ++++++++++++++-------------
>  include/linux/io-pgtable.h     |  8 ++++++++
>  2 files changed, 22 insertions(+), 13 deletions(-)

[...]

> 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)

This feels a bit fragile to me:
  * IOMMU-API users of io-pgtable shouldn't be passing this quirk
    but might end up doing so to paper over driver bugs.

  * Low-level users of io-pgtable who expose page-table operations to
    userspace need to pass the quirk, but might well not bother because
    well-behaved userspace doesn't trigger the warning.

So overall, it's all a bit unsatisfactory. Is there a way we could have
the warnings only when invoked via the IOMMU API?

Will