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

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Date:   Wed, 29 Jun 2022 11:09:00 +1200
From:   Michael Schmitz <schmitzmic@...il.com>
To:     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

Hi Arnd,

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.

Cheers,

     Michael

>
>        Arnd