Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE_SIZE

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

On 04/03/2024 4:10 pm, Michael Kelley wrote:
> From: Will Deacon <will@kernel.org> Sent: Monday, March 4, 2024 8:02 AM
>>
>> Hi folks,
>>
>> On Mon, Mar 04, 2024 at 04:55:06PM +0100, Petr Tesařík wrote:
>>> On Mon, 4 Mar 2024 13:37:56 +0000
>>> Robin Murphy <robin.murphy@arm.com> wrote:
>>>> On 04/03/2024 11:00 am, Petr Tesařík wrote:
>>>> [...]
>>>>>> Here's my take on tying all the threads together. There are
>>>>>> four alignment combinations:
>>>>>>
>>>>>> 1. alloc_align_mask: zero; min_align_mask: zero
>>
>> Based on this ^^^ ...
>>
>>>>>> xen_swiotlb_map_page() and dma_direct_map_page() are #1 or #2
>>>>>> via swiotlb_map() and swiotlb_tbl_map_single()
>>>>>>
>>>>>> iommu_dma_map_page() is #3 and #4 via swiotlb_tbl_map_single()
>>>>>>
>>>>>> swiotlb_alloc() is #3, directly to swiotlb_find_slots()
>>>>>>
>>>>>> For #1, the returned physical address has no constraints if
>>>>>> the requested size is less than a page. For page size or
>>>>>> greater, the discussed historical requirement for page
>>>>>> alignment applies.
>>
>> ... and this ^^^ ...
>>
>>
>>> I believe this patch series is now good as is, except the commit
>>> message should make it clear that alloc_align_mask and min_align_mask
>>> can both be zero, but that simply means no alignment constraints.
>>
>> ... my (possibly incorrect!) reading of the thread so far is that we
>> should preserve page-aligned allocation in this case if the allocation
>> size is >= PAGE_SIZE.
>>
>> Something like the diff below, to replace this final patch?
>>
>> Will
>>
>> --->8
>>
>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>> index c381a7ed718f..67eac05728c0 100644
>> --- a/kernel/dma/swiotlb.c
>> +++ b/kernel/dma/swiotlb.c
>> @@ -992,6 +992,14 @@ static int swiotlb_search_pool_area(struct device
>> *dev, struct io_tlb_pool *pool
>>          BUG_ON(!nslots);
>>          BUG_ON(area_index >= pool->nareas);
>>
>> +       /*
>> +        * Historically, allocations >= PAGE_SIZE were guaranteed to be
>> +        * page-aligned in the absence of any other alignment requirements.
>> +        * Since drivers may be relying on this, preserve the old behaviour.
>> +        */
>> +       if (!alloc_align_mask && !iotlb_align_mask && alloc_size >= PAGE_SIZE)
>> +               alloc_align_mask = PAGE_SIZE - 1;
>> +
> 
> Yes, I think that should do it.

In principle it might be more logical to fudge this into 
iotlb_align_mask rather than alloc_align_mask - since that's really the 
effective behaviour to preserve for streaming mappings - and then pass 
an explicit alloc_align_mask from swiotlb_alloc() to honour the 
dma-coherent requirements. However I also wouldn't really object to not 
going that far and instead just making the comment a bit clearer that 
this is still serving both purposes.

Cheers,
Robin.

> 
> Michael
> 
>>          /*
>>           * Ensure that the allocation is at least slot-aligned and update
>>           * 'iotlb_align_mask' to ignore bits that will be preserved when
>> @@ -1006,13 +1014,6 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
>>           */
>>          stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
>>
>> -       /*
>> -        * For allocations of PAGE_SIZE or larger only look for page aligned
>> -        * allocations.
>> -        */
>> -       if (alloc_size >= PAGE_SIZE)
>> -               stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
>> -
>>          spin_lock_irqsave(&area->lock, flags);
>>          if (unlikely(nslots > pool->area_nslabs - area->used))
>>                  goto not_found;
>