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

[Ard Biesheuvel]

On Fri, 15 Apr 2022 at 11:51, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 15 Apr 2022 at 10:12, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, Apr 15, 2022 at 10:05:21AM +0200, Ard Biesheuvel wrote:
> > >
> > > I guess that should be fixable. GIven that this is about padding
> > > rather than alignment, we could do something like
> > >
> > > struct crypto_request {
> > >   union {
> > >       struct {
> > >         ... fields ...
> > >       };
> > >       u8 __padding[ARCH_DMA_MINALIGN];
> > >    };
> > >     void __ctx[]  __align(CRYPTO_MINALIGN);
> > > };
> > >
> > > And then hopefully, we can get rid of the padding once we fix drivers
> > > doing non-cache coherent inbound DMA into those structures.
> >
> > Sorry, I don't think this works.  kmalloc can still return something
> > that's not ARCH_DMA_MINALIGN-aligned, and therefore __ctx won't be
> > aligned correctly.
> >
>
> That is the whole point, really: ARCH_DMA_MINALIGN==128 does not mean
> __ctx needs to be aligned to 128 bytes, it only means that it should
> not share a 128 byte cacheline with the preceding fields.

Let's rephrase that as 'must not share a cacheline with the preceding
fields, and the worst case we expect to have to deal with is a
cacheline size of 128 bytes'