linux-kernel - Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary

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Date: Fri, 22 Mar 2024 18:51:38 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Will Deacon <will@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Marek Szyprowski
 <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, Petr
 Tesarik <petr.tesarik1@...wei-partners.com>, Michael Kelley
 <mhklinux@...look.com>, open list <linux-kernel@...r.kernel.org>, "open
 list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>, Roberto Sassu
 <roberto.sassu@...weicloud.com>
Subject: Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary

On Fri, 22 Mar 2024 15:09:41 +0000
Will Deacon <will@...nel.org> wrote:

> Hi Petr,
> 
> On Thu, Mar 21, 2024 at 06:19:00PM +0100, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> > 
> > If the allocation alignment is bigger than IO_TLB_SIZE and min_align_mask
> > covers some bits in the original address between IO_TLB_SIZE and
> > alloc_align_mask, preserve these bits by allocating additional padding
> > slots before the actual swiotlb buffer.  
> 
> Thanks for fixing this! I was out at a conference last week, so I didn't
> get very far with it myself, but I ended up in a pickle trying to avoid
> extending 'struct io_tlb_slot'. Your solution is much better than the
> crazy avenue I started going down...
> 
> With your changes, can we now simplify swiotlb_align_offset() to ignore
> dma_get_min_align_mask() altogether and just:
> 
> 	return addr & (IO_TLB_SIZE - 1);

I have also thought about this but I don't think it's right. If we
removed dma_get_min_align_mask() from swiotlb_align_offset(), we would
always ask to preserve the lowest IO_TLB_SHIFT bits. This may cause
less efficient use of the SWIOTLB.

For example, if a device does not specify any min_align_mask, it is
presumably happy with any buffer alignment, so SWIOTLB may allocate at
the beginning of a slot, like here:

orig_addr   |      ++|++      |
tlb_addr    |++++    |        |

Without dma_get_min_align_mask() in swiotlb_align_offset(), it would
have to allocate two mostly-empty slots:

tlb_addr    |      ++|++      |

where:
  | mark a multiple of IO_TLB_SIZE (in physical address space)
  + used memory
    free memory

Petr T