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

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Date:   Thu, 7 Apr 2022 10:25:10 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN

On Thu, Apr 07, 2022 at 02:14:15PM +0800, Muchun Song wrote:
> On Tue, Apr 05, 2022 at 02:57:55PM +0100, Catalin Marinas wrote:
> > ARCH_DMA_MINALIGN represents the minimum (static) alignment for safe DMA
> > operations while ARCH_KMALLOC_MINALIGN is the minimum kmalloc() objects
> > alignment.
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> > Cc: Herbert Xu <herbert@...dor.apana.org.au>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > ---
> >  include/linux/crypto.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/crypto.h b/include/linux/crypto.h
> > index 2324ab6f1846..654b9c355575 100644
> > --- a/include/linux/crypto.h
> > +++ b/include/linux/crypto.h
> > @@ -167,7 +167,7 @@
> >   * maintenance for non-coherent DMA (cache invalidation in particular) does not
> >   * affect data that may be accessed by the CPU concurrently.
> >   */
> > -#define CRYPTO_MINALIGN ARCH_KMALLOC_MINALIGN
> > +#define CRYPTO_MINALIGN ARCH_DMA_MINALIGN
> 
> I don't think this should be changed since ARCH_KMALLOC_MINALIGN is
> already aligned with the size what you need.

With this series, ARCH_KMALLOC_MINALIGN is no longer safe for
non-coherent DMA on all arm64 SoCs, that's what ARCH_DMA_MINALIGN will
cover.

Now, looking at the comment for CRYPTO_MINALIGN, one aspect it covers is
the minimum alignment required by C for the crypto_tfm structure access.
So a smaller ARCH_KMALLOC_MINALIGN would do. But the other part of the
comment mentions in-structure alignment for non-coherent DMA. Here we'd
need the upper bound alignment, ARCH_DMA_MINALIGN.

I'll follow up on Herbert's email as I think he has a good point on
structure vs kmalloc() alignment.

-- 
Catalin