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

[Muchun Song]

On Thu, Apr 7, 2022 at 5:25 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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@xxxxxxx>
> > > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > > ---
> > >  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?  I think you are on the right road since most CPUs have
a 64-byte cache line.

Thanks.