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Date:   Tue, 28 Jun 2022 16:50:09 -0700
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Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
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To:     Michael Schmitz <schmitzmic@gmail.com>,
        Arnd Bergmann <arnd@kernel.org>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        scsi <linux-scsi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Jakub Kicinski <kuba@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        Khalid Aziz <khalid@gonehiking.org>,
        "Maciej W . Rozycki" <macro@orcam.me.uk>,
        Matt Wang <wwentao@vmware.com>,
        Miquel van Smoorenburg <mikevs@xs4all.net>,
        Mark Salyzyn <salyzyn@android.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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        Denis Efremov <efremov@linux.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
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From:   Bart Van Assche <bvanassche@acm.org>
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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@gmail.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.