Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Apr 07, 2022 at 06:00:10PM +0800, Muchun Song wrote:
> On Thu, Apr 7, 2022 at 5:25 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > 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@arm.com>
> > > > Cc: Herbert Xu <herbert@gondor.apana.org.au>
> > > > Cc: "David S. Miller" <davem@davemloft.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.
> 
> Got it. Now I know what you want to do. You want to set
> ARCH_KMALLOC_MINALIGN to 64, however, the smallest
> size of kmem_cache depends on the cache line size at
> runtime.  But we have to know the safe alignment at building
> time.  So we have to make those align with ARCH_DMA_MINALIGN.
> Right?  

Right.

> I think you are on the right road since most CPUs have a 64-byte cache
> line.

Yeah, apart from about three SoCs with 128, the rest use 64-byte cache
lines.

Longer term we should try reduce such alignment below the cache line
size *if* the SoC fully DMA-coherent (like x86). In that case cache
maintenance for DMA doesn't exist, so there's no risk to smaller
allocations. Of course, one can use a kmem_cache_create() with
SLAB_HWCACHE_ALIGN and get the cacheline alignment for performance
reasons.

-- 
Catalin