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

Robin Murphy
2024-03-05T11:20:17Z

On 2024-03-04 6:22 pm, Michael Kelley wrote:
> From: Robin Murphy <robin.murphy@arm.com> Sent: Monday, March 4, 2024 8:54 AM
>>
>> 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
>
> I'm not understanding what you are getting at, but maybe we are
> interpreting the historical page alignment requirement differently.
> I think of the page alignment requirement as independent of the
> orig_addr -- the returned physical address should always be exactly
> page aligned, and not offset to match bits in orig_addr. If that's
> the case, then implementing the page alignment via
> alloc_align_mask is logically the right place. Fudging into
> iotlb_align_mask would do matching of bits in orig_addr.
>
> Or is there something else I'm not considering?

In short, it's that alloc_align_mask is concerned with how slots are
allocated, while min_align_mask is concerned with where the data itself
is bounced (which may also place certain constraints on allocation).

The reason this page-alignment was here in the first place was seemingly
to serve the original swiotlb_alloc() path, and thus it could be
considered functionally equivalent to what is now alloc_align_mask.
However the side-effect it happened to also have for streaming mappings
was to prevent sufficiently large page-aligned buffers being bounced to
a non-page-aligned address, which apparently managed to work well enough
for NVMe until 64K pages became more common and ruined things by being
too big, and we formalised *that* desired effect into min_align_mask.

I get that forcing io_tlb_align mask here would introduce a stronger
constraint which affects non-page-aligned buffers as well, and wanting
to avoid that is perhaps a reasonable concern, so once again I'm really
just arguing semantics. Given the head-scratching we've been through
over this already, I think it would be valuable to at least be clearer
that a significant part of the "old behaviour" is to do the right thing
for swiotlb_alloc(), which is very much still current and necessary, but
the reason we're not setting alloc_align_mask there is because doing it
here also preserves this legacy side-effect which acts as a limited
version of min_align_mask to preserve page alignment when bouncing.

Cheers,
Robin.