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

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

On 2025-05-20 12:31 pm, Will Deacon wrote:
> On Thu, May 15, 2025 at 07:48:39AM -0700, Rob Clark wrote:
>> On Thu, May 15, 2025 at 7:33 AM Will Deacon <will@kernel.org> wrote:
>>>
>>> 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?
>>
>> iommu drivers _not_ setting this flag seems like a good way to achieve that ;-)
>>
>> The alternative is to move the warns to the iommu driver... but they
>> could just as easily remove the WARN_ON()s as they could set the
>> NO_WARN_ON quirk, so :shrug:?
> 
> Bah, I also don't have a good idea to improve this, so I guess I'll take
> what you have for now.

Hmm, just a nit on reflection, how about fixing up the name to just 
IO_PGTABLE_QUIRK_NO_WARN? Given that it's already quite long, and we 
have a well-established DMA_ATTR_NO_WARN with equivalent semantics over 
in the DMA API.

Cheers,
Robin.