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

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

On 7/3/2026 9:42 PM, Jason Gunthorpe wrote:
> On Thu, Jun 18, 2026 at 02:32:09PM +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, the CONT bit can be set on all entries in the group to allow
>> the hardware to combine them into a single TLB entry, improving TLB
>> utilization.
>>
>> The contiguous hint sizes per granule are:
>>
>>   Page Size | CONT PTE |  PMD  | CONT PMD
>>   ----------+----------+-------+---------
>>       4K    |   64K    |   2M  |   32M
>>      16K    |    2M    |  32M  |    1G
>>      64K    |    2M    | 512M  |   16G
> 
> My series to convert smmuv3 to the iommupt takes care of this and
> supports all the orders too. I'd rather we move forward with that then
> try to patch up this.
> 

Thanks for details, I have gone through your series. As this patch
targets io-pgtable-arm.c directly and would benefit all its users (SMMUv2,
Apple DART, etc.), not just SMMUv3. I think there will still be value in
this patch for the other users.

>> Support is gated behind CONFIG_IOMMU_IO_PGTABLE_CONTIG_HINT, which
>> provides a compile-time opt-out for hardware affected by SMMU errata
>> related to the contiguous bit.
> 
> I reviewed the errata and didn't find any related to contig that
> required disabling contig, the driver was OK with the ones I
> found.. Did you find something specific?
> 
>> On the unmapping side, the CONT bit is cleared from all PTEs in the
>> affected contiguous group before any individual entry is invalidated,
>> following the Break-Before-Make requirement of the architecture.
> 
> BBM means you make it non-present and flush, it doesn't mean you clear
> CONT, this is wrong..
> 
> Jason

You're right. The arm_lpae_cont_clear() step was introduced to support
partial unmaps of a contiguous group — clearing CONT first so that
individual entries could then be invalidated one by one. However, as
Daniel also pointed out, partial unmap of a contiguous group should not
be supported at all. Since v2 will enforce that an unmap must exactly
match the full aligned contiguous range, arm_lpae_cont_clear() is no
longer needed and will be removed. The unmap path will simply write
invalid entries for all PTEs in the contiguous group and issue a single
range TLB flush covering the entire range thereby following BBM.

Thanks,
Vijay