On 6/28/22 16:09, Michael Schmitz wrote:

On 29/06/22 09:50, Arnd Bergmann wrote:

quoted

On Tue, Jun 28, 2022 at 11:03 PM Michael Schmitz 
[off-list ref] wrote:

quoted

On 28/06/22 19:03, Geert Uytterhoeven wrote:

quoted

quoted

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.