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Message-ID: <fc47e8da-81d3-e563-0a17-4eb23db015cc@acm.org>
Date: Tue, 28 Jun 2022 16:50:09 -0700
From: Bart Van Assche <bvanassche@....org>
To: Michael Schmitz <schmitzmic@...il.com>,
Arnd Bergmann <arnd@...nel.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
scsi <linux-scsi@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
Jakub Kicinski <kuba@...nel.org>,
Christoph Hellwig <hch@...radead.org>,
Marek Szyprowski <m.szyprowski@...sung.com>,
Robin Murphy <robin.murphy@....com>,
Linux IOMMU <iommu@...ts.linux-foundation.org>,
Khalid Aziz <khalid@...ehiking.org>,
"Maciej W . Rozycki" <macro@...am.me.uk>,
Matt Wang <wwentao@...are.com>,
Miquel van Smoorenburg <mikevs@...all.net>,
Mark Salyzyn <salyzyn@...roid.com>,
linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
Linux-Arch <linux-arch@...r.kernel.org>,
alpha <linux-alpha@...r.kernel.org>,
linux-m68k <linux-m68k@...ts.linux-m68k.org>,
Parisc List <linux-parisc@...r.kernel.org>,
Denis Efremov <efremov@...ux.com>,
Michael Ellerman <mpe@...erman.id.au>,
John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
On 6/28/22 16:09, Michael Schmitz wrote:
> On 29/06/22 09:50, Arnd Bergmann wrote:
>> On Tue, Jun 28, 2022 at 11:03 PM Michael Schmitz
>> <schmitzmic@...il.com> wrote:
>>> On 28/06/22 19:03, Geert Uytterhoeven wrote:
>>>>> The driver allocates bounce buffers using kmalloc if it hits an
>>>>> unaligned data buffer - can such buffers still even happen these days?
>>>> No idea.
>>> Hmmm - I think I'll stick a WARN_ONCE() in there so we know whether this
>>> code path is still being used.
>> kmalloc() guarantees alignment to the next power-of-two size or
>> KMALLOC_MIN_ALIGN, whichever is bigger. On m68k this means it
>> is cacheline aligned.
>
> And all SCSI buffers are allocated using kmalloc? No way at all for user
> space to pass unaligned data?
>
> (SCSI is a weird beast - I have used a SCSI DAT tape driver many many
> years ago, which broke all sorts of assumptions about transfer block
> sizes ... but that might actually have been in the v0.99 days, many
> rewrites of SCSI midlevel ago).
>
> Just being cautious, as getting any of this tested will be a stretch.
An example of a user space application that passes an SG I/O data buffer
to the kernel that is aligned to a four byte boundary but not to an
eight byte boundary if the -s (scattered) command line option is used:
https://github.com/osandov/blktests/blob/master/src/discontiguous-io.cpp
Bart.
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