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

Robin Murphy
2024-03-04T13:38:00Z

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
>> 2. alloc_align_mask: zero; min_align_mask: non-zero
>> 3. alloc_align_mask: non-zero; min_align_mask: zero/ignored
>> 4. alloc_align_mask: non-zero; min_align_mask: non-zero
>
> What does "min_align_mask: zero/ignored" mean? Under which
> circumstances should be a non-zero min_align_mask ignored?
>
>> 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.
>>
>> For #2, min_align_mask governs the bits of the returned
>> physical address that must match the original address. When
>> needed, swiotlb must also allocate pre-padding aligned to
>> IO_TLB_SIZE that precedes the returned physical address. A
>> request size <= swiotlb_max_mapping_size() will not exceed
>> IO_TLB_SEGSIZE even with the padding. The historical
>> requirement for page alignment does not apply because the
>> driver has explicitly used the newer min_align_mask feature.
>
> What is the idea here? Is it the assumption that only old drivers rely
> on page alignment, so if they use min_align_mask, it proves that they
> are new and must not rely on page alignment?

Yes, if a driver goes out of its way to set a min_align_mask which is
smaller than its actual alignment constraint, that is clearly the
driver's own bug. Strictly we only need to be sympathetic to drivers
which predate min_align_mask, when implicitly relying on page alignment
was all they had.

>> For #3, alloc_align_mask specifies the required alignment. No
>> pre-padding is needed. Per earlier comments from Robin[1],
>> it's reasonable to assume alloc_align_mask (i.e., the granule)
>> is >= IO_TLB_SIZE. The original address is not relevant in
>> determining the alignment, and the historical page alignment
>> requirement does not apply since alloc_align_mask explicitly
>> states the alignment.

FWIW I'm also starting to wonder about getting rid of the alloc_size
argument and just have SWIOTLB round the end address up to
alloc_align_mask itself as part of all these calculations. Seems like it
could potentially end up a little simpler, maybe?

>> For #4, the returned physical address must match the bits
>> in the original address specified by min_align_mask. swiotlb
>> swiotlb must also allocate pre-padding aligned to
>> alloc_align_mask that precedes the returned physical address.
>> Also per Robin[1], assume alloc_align_mask is >=
>> min_align_mask, which solves the conflicting alignment
>> problem pointed out by Petr[2]. Perhaps we should add a
>> "WARN_ON(alloc_align_mask < min_align_mask)" rather than
>> failing depending on which bits of the original address are
>> set. Again, the historical requirement for page alignment does
>> not apply.
>
> AFAICS the only reason this works in practice is that there are only
> two in-tree users of min_align_mask: NVMe and Hyper-V. Both use a mask
> of 12 bits, and the IOVA granule size is never smaller than 4K.

If we assume a nonzero alloc_align_mask exclusively signifies iommu-dma,
then for this situation SWIOTLB should only need to worry about the
intersection of alloc_align_mask & min_align_mask, since any
min_align_mask bits larger than the IOVA granule would need to be
accounted for in the IOVA allocation regardless of SWIOTLB.
> If we want to rely on this, then I propose to make a BUG_ON() rather
> than WARN_ON().

I've just proposed a patch to make it not matter for now - the nature of
iommu-dma makes it slightly more awkward to prevent SWIOTLB from ever
seeing this condition at all, so I chose not to do that, but as long as
swiotlb_tbl_map_single() does *something* for conflicting constraints
without completely falling over, which swiotlb_tbl_unmap_single can then
undo again, then it should be fine.

Thanks,
Robin.