linux-kernel - Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE

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Message-ID: <20240229133346.GA7177@lst.de>
Date: Thu, 29 Feb 2024 14:33:46 +0100
From: Christoph Hellwig <hch@....de>
To: Michael Kelley <mhklinux@...look.com>
Cc: Will Deacon <will@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	"kernel-team@...roid.com" <kernel-team@...roid.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Dexuan Cui <decui@...rosoft.com>,
	Nicolin Chen <nicolinc@...dia.com>
Subject: Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE

On Thu, Feb 29, 2024 at 07:36:05AM +0000, Michael Kelley wrote:
> If there *is* a requirement for page alignment of page-size-or-greater
> requests, even when alloc_align_mask and min_align_mask are zero,
> we need to think about how to do that correctly, as that requirement
> is no longer met after Patch 1 of this series.

It has been historical behavior that all dma allocations are page
aligned (including in fact smaller than page sized ones that get
rounded up to a page size).  The documentation actually (incorretly)
states an even stronger guarantee:

"The CPU virtual address and the DMA address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size.  This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary."