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

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Date: Wed, 27 Mar 2024 19:42:01 +0000
From: Will Deacon <will@...nel.org>
To: Petr Tesařík <petr@...arici.cz>
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, Mar 22, 2024 at 06:51:38PM +0100, Petr Tesařík wrote:
> 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

Thanks for the patient explanation. I'd got so caught up with the DMA
alignment mask that I forgot the usual case where it's not specified at
all!

Will