Re: [PATCH v5] iommu/io-pgtable-arm: Add support for contiguous hint bit

[Vijayanand Jitta <vijayanand.jitta@oss.qualcomm.com>]

On 9/25/2026 4:23 AM, Jason Gunthorpe wrote:
> On Thu, Sep 24, 2026 at 11:49:50AM -0700, Daniel Mentz wrote:
>> On Wed, Sep 23, 2026 at 5:15 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>>>
>>> On Mon, Sep 21, 2026 at 04:44:07PM +0530, Vijayanand Jitta wrote:
>>>> From: Prakash Gupta <prakash.gupta@oss.qualcomm.com>
>>>>
>>>> Add support for the contiguous hint (CONT) bit in ARM LPAE page tables.
>>>> When a set of consecutive PTEs map a naturally aligned contiguous block of
>>>> memory, set CONT on every descriptor in that group so the hardware can
>>>> combine translations and improve TLB reach.
>>>>
>>>> Advertise the supported CONT group sizes in pgsize_bitmap. Callers select
>>>> those sizes through the normal page-size selection path; io-pgtable-arm
>>>> then installs the corresponding tagged descriptors directly. A partial
>>>> unmap of a tagged CONT group is rejected before modifying any descriptor,
>>>> so a rejected request cannot leave the group partly unmapped.
>>>>
>>>> The IO_PGTABLE_QUIRK_ARM_NO_CONT_HINT quirk allows SMMU drivers to disable
>>>> CONT support for hardware with implementation-specific errata.
>>>
>>> smmuv3 has this errata, it must be disabled there too. I didn't notice
>>> it in this patch?
>>
>> SMMU is an architecture specification. I am not aware of errors in
>> this architecture specification that would preclude the usage of the
>> contiguous bit. I understand that Arm MMU-700 has the following
>> erratum
>>
>> 3777127 Under invalidation in TBU possible when using contiguous page
>> table entries
> 
> And a neoverse one too.
> 
>> The recommended workaround is described as
>>
>> "Ensure that contiguous page tables are removed using a single range
>> invalidation. Arm recommends using range invalidations to remove
>> contiguous entries anyway for performance reasons."
>>
>> and I believe we are already doing this.
> 
> No we aren't. Go read my fix on this:
> 
> https://lore.kernel.org/linux-iommu/1-v7-e84261bbe7cd+2ea80b-smmu_tlbi_jgg@nvidia.com/
> 
> I have another patch that fixes it for iommu domain mappings too.
> 
> Jason

Sure , will disable it for smmuv3 for now.

Thanks,
Vijay