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

Catalin Marinas
2022-04-17T16:29:12Z

On Sun, Apr 17, 2022 at 04:43:33PM +0800, Herbert Xu wrote:
> On Sun, Apr 17, 2022 at 09:38:40AM +0100, Catalin Marinas wrote:
> > I don't think we need to do anything here. A structure like:
> >
> > struct x {
> > char y;
> > char z[] CRYPTO_MINALIGN_ATTR;
> > };
> >
> > is already of size 128. Without CRYPTO_MINALIGN_ATTR, its size would be
> > 1 but otherwise the whole structure inherits the alignment of its
> > member and this translates into an aligned size.
>
> No we should not lie to the compiler,

We won't if we ensure that a structure with sizeof() >= 128 is aligned
to 128.

> we have code elsewhere
> that uses the alignment to compute the amount of extra padding
> needed to create greater padding. If CRYPTO_MINALIGN is misleading
> then that calculation will fall apart.

There is no direct CRYPTO_MINALIGN use for any extra padding AFAICT.
There is an indirect use via __alignof__(ctx) like in
crypto_tfm_ctx_alignment() but IIUC in that case CRYPTO_MINALIGN is a
statement of what you want rather than what you get from kmalloc(). So
if you want 128 alignment of tfm->__crt_ctx, you should say so by either
changing the attribute to __aligned(ARCH_DMA_MINALIGN) or keeping
CRYPTO_MINALIGN as 128.

There is the union padding that Ard suggested but I don't think it buys
us much, the __aligned(ARCH_DMA_MINALIGN) gives you the padding and the
kmalloc() rules the alignment (subject to removing kmalloc-192). The
code that allocates these would need to place the structure aligned
anyway, irrespective of whether we use the padding or the
__aligned(ARCH_DMA_MINALIGN).

> So keep CRYPTO_MINALIGN at whatever alignment you lower kmalloc
> to, and then add the padding you need to separate the Crypto API
> bits from the context. In fact, that is exactly what cra_alignmask
> is supposed to do.

I disagree on the cra_alignmask intention here, though I may be wrong as
I did not write the code. Yes, you could make it ARCH_DMA_MINALIGN
everywhere but IMHO that's not what it is supposed to do. The driver
only knows about the bus master alignment requirements (typically
smaller than a cache line) while the architecture-defined
ARCH_DMA_MINALIGN cares about the non-coherent DMA requirements.

> Sure we currently limit the maximum alignment to 64 bytes because
> of stack usage but we can certainly look into increasing it as
> that's what you're doing here anyway.

I'm not actually increasing CRYPTO_MINALIGN, just trying to keep it as
the current value of 128 for arm64.

--
Catalin