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

From: Douglas Anderson
Date: Mon Sep 21 2020 - 20:26:31 EST

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.

Trying to super-optimize the xor operation makes a lot of sense if
you're using software RAID, but the above is probably not worth it for
most Linux users because:
1. Quite a few arm64 kernels are built for embedded systems where
software raid isn't common. That means we're spending lots of time
on every boot trying to optimize something we don't use.
2. Presumably, if we have neon, it's faster than alternatives. If
it's not, it's not expected to be tons slower.
3. Quite a lot of arm64 systems are big.LITTLE. This means that the
existing test is somewhat misguided because it's assuming that test
results on the boot CPU apply to the other CPUs in the system.
This is not necessarily the case.

Let's add a new config option that allows us to just use the neon
functions (if present) without benchmarking.

NOTE: One small side effect is that on an arm64 system _without_ neon
we'll end up testing the xor_block_8regs_p and xor_block_32regs_p
versions of the function. That's presumably OK since we already test
all those when KERNEL_MODE_NEON is disabled.

ALSO NOTE: presumably the way to do better than this is to add some
sort of per-CPU-core lookup table and jump to a per-CPU-core-specific
XOR function each time xor is called. Without seeing evidence that
this would really help someone, though, that doesn't seem worth it.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

arch/arm64/Kconfig | 15 +++++++++++++++
arch/arm64/include/asm/xor.h | 5 +++++
2 files changed, 20 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 64ae5e4eb814..fc18df45a5f8 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -306,6 +306,21 @@ config SMP
def_bool y

+ bool "Assume neon is fastest for xor if the CPU supports it"
+ default y
+ depends on KERNEL_MODE_NEON
+ help
+ Normally the kernel will run through several different XOR
+ algorithms at boot, timing them on the boot processor to see
+ which is fastest. This can take quite some time. On many
+ machines it's expected that, if NEON is available, it's going
+ to provide the fastest implementation. If you set this option
+ we'll skip testing this every boot and just assume NEON is the
+ fastest if present. Setting this option will speed up your
+ boot but you might end up with a less-optimal xor
+ implementation.
def_bool y

diff --git a/arch/arm64/include/asm/xor.h b/arch/arm64/include/asm/xor.h
index 947f6a4f1aa0..1acb290866ab 100644
--- a/arch/arm64/include/asm/xor.h
+++ b/arch/arm64/include/asm/xor.h
@@ -57,6 +57,10 @@ static struct xor_block_template xor_block_arm64 = {
.do_4 = xor_neon_4,
.do_5 = xor_neon_5
+ (cpu_has_neon() ? &xor_block_arm64 : FASTEST)
do { \
@@ -66,5 +70,6 @@ static struct xor_block_template xor_block_arm64 = {
} \
} while (0)