On 11/14/18 5:47 PM, Arnd Bergmann wrote: > On Tue, Nov 13, 2018 at 2:58 AM Vincenzo Frascino > <vincenzo.frascino@xxxxxxx> wrote: >> >> On 09/11/2018 16:13, Arnd Bergmann wrote: > >>>> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S >>>> index beca249bc2f3..9de0ffc369c5 100644 >>>> --- a/arch/arm64/kernel/vdso/vdso.lds.S >>>> +++ b/arch/arm64/kernel/vdso/vdso.lds.S >>>> @@ -88,6 +88,7 @@ VERSION >>>> __kernel_gettimeofday; >>>> __kernel_clock_gettime; >>>> __kernel_clock_getres; >>>> + __kernel_time; >>>> local: *; >>>> }; >>>> } >>> >>> I would prefer to not add any deprecated interfaces in the VDSO. If we >>> have the 64-bit version of clock_gettime, we don't need the 32-bit version >>> of it, and we don't need gettimeofday() or time() either. The C library >>> can easily implement those by calling into clock_gettime. >>> >> >> I like the idea, this would make the vdso lib code more simple and more maintainable. >> >> In this patchset I tried to cover the widest possible scenario making things configurable: each architecture can select and enable exactly what it needs from the vdso common code. >> >> Based on what you are proposing, once the C library will implement things in this way, it will be easy to deprecate and remove the unused code. > > Just to clarify: we can never remove interfaces that an older version of the > C library was using. What I'm asking is that we don't introduce any of the > unnecessary ones for architectures that don't already have them. > I agree, I realize now I should have worded my answer differently. With "deprecate and remove the unused code" was referring to redirect gettimeofday through clock_gettime, not to remove completely gettimeofday interface since this would cause userspace breakage for old C libraries as you are pointing out as well. >> I am not familiar with the development plans of the various C libraries, but looking at >> bionic libc currently seems using all the vdso exposed functions [1]. >> >> [1] https://github.com/aosp-mirror/platform_bionic/blob/master/libc/bionic/vdso.cpp > > It looks like this implementation checks for each one of them to be present > first and then uses a fallback implementation if it does not exist. This would > clearly let us remove the handlers we don't want to support, but there > are two possible downsides: >> - some other libc might be lacking that fallback path > - the fallback might be much slower, e.g. time() should fallback to > the vdso version of clock_gettime(CLOCK_REALTIME_COARSE) for > best performance, rather than the time() syscall or CLOCK_REALTIME. > > So I'd argue that if an architecture already has a time() vdso implementation, > we probably want to keep that entry point but make it point to the best > generic implementation (i.e. clock_gettime CLOCK_REALTIME_COARSE). What I was trying to point out here is that if the symbol is present in the vdso library the C library implementation tends to prefer to use it. Said that, I agree with what you are saying and in this first iteration I will make sure that we do not add new symbols for the architectures that did not support them previously, but I will keep the library the most generic possible, I would prefer though to introduce the redirection for gettimeofday() and time() to the best performing clock_gettime() at a later stage (and with a separate patchset) because it would involve some cleanup in the vdso.c of the architectures that would use the library (i.e. update_vsyscall_tz() not required anymore) and because based on what I measured with a quick test the performance difference seems small with the current implementation of the library. > > Arnd > -- Regards, Vincenzo
