linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Message-ID: <CAMj1kXFOn7F1bfwn_xDGvk3dKt8UwmPcpemzXds33eYHVCgR-Q@mail.gmail.com>
Date:   Thu, 21 Apr 2022 10:05:49 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

On Thu, 21 Apr 2022 at 09:20, Christoph Hellwig <hch@...radead.org> wrote:
>
> Btw, there is another option:  Most real systems already require having
> swiotlb to bounce buffer in some cases.  We could simply force bounce
> buffering in the dma mapping code for too small or not properly aligned
> transfers and just decrease the dma alignment.

Strongly agree. As I pointed out before, we'd only need to do this for
misaligned, non-cache coherent inbound DMA, and we'd only have to
worry about performance regressions, not data corruption issues. And
given the natural alignment of block I/O, and the fact that network
drivers typically allocate and map their own RX buffers (which means
they could reasonably be fixed if a performance bottleneck pops up), I
think the risk for showstopper performance regressions is likely to be
acceptable.