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

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Date:   Fri, 15 Apr 2022 12:04:11 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        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 Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

On Fri, 15 Apr 2022 at 11:51, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Fri, 15 Apr 2022 at 10:12, Herbert Xu <herbert@...dor.apana.org.au> 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'