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Message-ID: <YzseX7suH3t5nlT6@arm.com>
Date: Mon, 3 Oct 2022 18:39:43 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ard Biesheuvel <ardb@...nel.org>,
Isaac Manjarres <isaacmanjarres@...gle.com>,
Herbert Xu <herbert@...dor.apana.org.au>,
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>,
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>,
Saravana Kannan <saravanak@...gle.com>, kernel-team@...roid.com
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
ARCH_KMALLOC_MINALIGN
On Sun, Oct 02, 2022 at 03:24:57PM -0700, Linus Torvalds wrote:
> On Sun, Oct 2, 2022 at 3:09 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > Non-coherent DMA for networking is going to be fun, though.
>
> I agree that networking is likely the main performance issue, but I
> suspect 99% of the cases would come from __alloc_skb().
The problem is not the allocation but rather having a generic enough
dma_needs_bounce() check. It won't be able to tell whether some 1500
byte range is for network or for crypto code that uses a small
ARCH_KMALLOC_MINALIGN. Getting the actual object size (e.g. with
ksize()) doesn't tell the full story on how safe the DMA is.
> Similarly, that code already has magic stuff to try to be
> cacheline-aligned for accesses, but it's not really for DMA coherency
> reasons, just purely for performance reasons (trying to make sure that
> the header accesses stay in one cacheline etc).
Yeah, __skb_alloc() ends up using SMP_CACHE_BYTES for data alignment
(via SKB_DATA_ALIGN). I have a suspicion this may break on SoCs with a
128-byte cache line but I haven't seen any report yet (there aren't many
such systems).
--
Catalin
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