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 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). Arnd
