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

[Bart Van Assche]

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@xxxxxxxxx> 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.