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

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Hi Bart,

On 29/06/22 11:50, Bart Van Assche wrote:
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

Thanks - four byte alignment actually wouldn't be an issue for me. It's two byte or smaller that would trip up the SCSI DMA.

While I'm sure such an even more pathological test case could be written, I was rather worried about st.c and sr.c input ...

Cheers,

    Michael


Bart.



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