linux-kernel - Re: [PATCH V8 4/5] libsas: Align SMP req/resp to dma_get_cache

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Date:   Thu, 19 Oct 2017 17:12:14 +0200
From:   Christoph Hellwig <hch@....de>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Huacai Chen <chenhc@...ote.com>, Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fuxin Zhang <zhangfx@...ote.com>, linux-kernel@...r.kernel.org,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <james.hogan@...tec.com>,
        linux-mips@...ux-mips.org,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        linux-ide@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH V8 4/5] libsas: Align SMP req/resp to
        dma_get_cache_alignment()

On Tue, Oct 17, 2017 at 01:55:43PM +0200, Marek Szyprowski wrote:
> If I remember correctly, kernel guarantees that each kmalloced buffer is
> always at least aligned to the CPU cache line, so CPU cache can be
> invalidated on the allocated buffer without corrupting anything else.

Yes, from slab.h:

/*
 * Some archs want to perform DMA into kmalloc caches and need a guaranteed
 * alignment larger than the alignment of a 64-bit integer.
 * Setting ARCH_KMALLOC_MINALIGN in arch headers allows that.
 */
#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
#define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
#else
#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
#endif

Mips sets this for a few subarchitectures, but it seems like you need
to set it for yours as well.