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

[Michael Kelley]

From: Will Deacon <will@xxxxxxxxxx> 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@xxxxxxx> 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.

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;