On 11/09/2018 04:29 PM, David Miller wrote:

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
Date: Fri,  9 Nov 2018 12:37:10 +0000

quoted

To take advantage of the commonly defined vdso interface for
gettimeofday the architectural code requires an adaptation.

This patch re-implements the gettimeofday vdso in C in order to use
lib/vdso.

With the new implementation arm64 gains support for CLOCK_BOOTTIME,
CLOCK_TAI and __kernel_time.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

What is the performance penalty for going to C from ASM like this?

I like the vDSO consolidation, but actually I think it makes a lot of
sense to implement these things in assembler with some C implementation
as guidance or for architectures that haven't optimized into an ASM
version yet.

I'd also like to see if we can somehow make PowerPC's optimizations in
this area usable in more places.

It defines a reciprocal in order to use a multiply instead of a divide
after capturing the time values.

I say all of this because I want to do something similar on sparc, and
having a generic implementation of all of the math would make that easier
for me and other architecture maintainers who might want to do the same.

I saw an _improvement_ in performance for the C implementation (for 
arm64), the optimizer did a better job with the specific processor 
architecture and actually killed a few bugs in the assembler 
implementation. The improvement was in the order of a nano-second or two 
in micro-benchmarks.

I used the bionic benchmarks to measure the performance.

-- Mark