Re: [PATCH 07/27] arm64: Substitute gettimeofday with C implementation

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

Hi David,

thank you for reviewing my code.

On 10/11/2018 00:29, David Miller wrote:
> From: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Date: Fri,  9 Nov 2018 12:37:10 +0000
> 
>> 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 <will.deacon@arm.com>
>> 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 did run some benchmarks on arm64 with both vdsotest-bench (I added a
script to the vdsotest implementation that can be used to replicate the
results) and an ad-hoc test that stresses the vdso library and the
differences in terms of performance penalty are very little (~1%) to
none in some cases.  

-- 
Regards,
Vincenzo