linux-kernel - Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO

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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.