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

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Message-ID: <YlVxGAHHD/j6lW3c@arm.com>
Date:   Tue, 12 Apr 2022 13:31:20 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        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 Tue, Apr 12, 2022 at 06:18:46PM +0800, Herbert Xu wrote:
> On Tue, Apr 12, 2022 at 11:02:54AM +0100, Catalin Marinas wrote:
> > This series does not penalise any architecture. It doesn't even make
> > arm64 any worse than it currently is.
> 
> Right, the patch as it stands doesn't change anything.  However,
> it is also broken as it stands.  As I said before, CRYPTO_MINALIGN
> is not something that is guaranteed by the Crypto API, it is simply
> a statement of whatever kmalloc returns.

I agree that CRYPTO_MINALIGN is not guaranteed by the Crypto API. What
I'm debating is the intended use for CRYPTO_MINALIGN in some (most?) of
the drivers. It's not just about kmalloc() but also a build-time offset
of buffers within structures to guarantee DMA safety. This can't be
fixed by cra_alignmask.

We could leave CRYPTO_MINALIGN as ARCH_KMALLOC_MINALIGN and that matches
it being just a statement of the kmalloc() minimum alignment. But since
it is also overloaded with the DMA in-structure offset alignment, we'd
need a new CRYPTO_DMA_MINALIGN (and _ATTR) to annotate those structures.
I have a suspicion there'll be fewer of the original CRYPTO_MINALIGN
uses left, hence my approach to making this bigger from the start.

There's also Ard's series introducing CRYPTO_REQ_MINALIGN while leaving
CRYPT_MINALIGN for DMA-safe offsets (IIUC):

https://lore.kernel.org/r/20220406142715.2270256-1-ardb@kernel.org

> So if kmalloc is no longer returning CRYPTO_MINALIGN-aligned
> memory, then those drivers that need this alignment for DMA
> will break anyway.

No. As per one of my previous emails, kmalloc() will preserve the DMA
alignment for an SoC even if smaller than CRYPTO_MINALIGN (or a new
CRYPTO_DMA_MINALIGN). Since kmalloc() returns DMA-safe pointers and
CRYPTO_MINALIGN (or a new CRYPTO_DMA_MINALIGN) is DMA-safe, so would an
offset from a pointer returned by kmalloc().

> If you want the Crypto API to guarantee alignment over and above
> that returned by kmalloc, the correct way is to use cra_alignmask.

For kmalloc(), this would work, but for the current CRYPTO_MINALIGN_ATTR
uses it won't.

Thanks.

-- 
Catalin