RE: [PATCH v2 1/4] lib: test copy_{to,from}_user()

David Laight
2023-03-23T22:16:29Z

From: Mark Rutland
> Sent: 22 March 2023 14:05
....
> > IIUC, in such tests you only vary the destination offset. Our copy
> > routines in general try to align the source and leave the destination
> > unaligned for performance. It would be interesting to add some variation
> > on the source offset as well to spot potential issues with that part of
> > the memcpy routines.
>
> I have that on my TODO list; I had intended to drop that into the
> usercopy_params. The only problem is that the cross product of size,
> src_offset, and dst_offset gets quite large.

I thought that is was better to align the writes and do misaligned reads.
Although maybe copy_to/from_user() would be best aligning the user address
(to avoid page faults part way through a misaligned access).

OTOH, on x86, is it even worth bothering at all.
I have measured a performance drop for misaligned reads, but it
was less than 1 clock per cache line in a test that was doing
2 misaligned reads in at least some of the clock cycles.
I think the memory read path can do two AVX reads each clock.
So doing two misaligned 64bit reads isn't stressing it.

David

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