linux-kernel - Re: [PATCH 1/2] xen/swiotlb: relax alignment requirements

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Message-ID: <5255102c-de9f-4cd8-8311-5d5b5eb26832@suse.com>
Date: Wed, 12 Feb 2025 07:53:13 +0100
From: Jan Beulich <jbeulich@...e.com>
To: Juergen Gross <jgross@...e.com>
Cc: Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 xen-devel@...ts.xenproject.org, Jan Vejvalka <jan.vejvalka@...otol.cuni.cz>,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Subject: Re: [PATCH 1/2] xen/swiotlb: relax alignment requirements

On 11.02.2025 13:04, Juergen Gross wrote:
> When mapping a buffer for DMA via .map_page or .map_sg DMA operations,
> there is no need to check the machine frames to be aligned according
> to the mapped areas size. All what is needed in these cases is that the
> buffer is contiguous at machine level.

Is this really true in all cases? Can't e.g. compound pages make it here,
with the caller then still being permitted to assume higher than page
alignment? Alignment checking in xen_swiotlb_map_page() would perhaps
need doing with the base address of the incoming page, i.e. excluding
the incoming offset.

Jan