Re: [PATCH] arm64: crypto: Add an option to assume NEON XOR is the fastest

[Ard Biesheuvel]

On Tue, 22 Sep 2020 at 10:26, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Douglas Anderson
> > Sent: 22 September 2020 01:26
> >
> > On every boot time we see messages like this:
> >
> > [    0.025360] calling  calibrate_xor_blocks+0x0/0x134 @ 1
> > [    0.025363] xor: measuring software checksum speed
> > [    0.035351]    8regs     :  3952.000 MB/sec
> > [    0.045384]    32regs    :  4860.000 MB/sec
> > [    0.055418]    arm64_neon:  5900.000 MB/sec
> > [    0.055423] xor: using function: arm64_neon (5900.000 MB/sec)
> > [    0.055433] initcall calibrate_xor_blocks+0x0/0x134 returned 0 after 29296 usecs
> >
> > As you can see, we spend 30 ms on every boot re-confirming that, yet
> > again, the arm64_neon implementation is the fastest way to do XOR.
> > ...and the above is on a system with HZ=1000.  Due to the way the
> > testing happens, if we have HZ defined to something slower it'll take
> > much longer.  HZ=100 means we spend 300 ms on every boot re-confirming
> > a fact that will be the same for every bootup.
>
> Can't the code use a TSC (or similar high-res counter) to
> see how long it takes to process a short 'hot cache' block?
> That wouldn't take long at all.
>

This is generic code that runs from an core_initcall() so I am not
sure we can easily implement this in a portable way.

Doug: would it help if we deferred this until late_initcall()? We
could take an arbitrary pick from the list at core_initcall() time to
serve early users, and update to the fastest one at a later time.