Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

Bart Van Assche
2022-06-28T23:50:21Z

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.