On Tue, Nov 13, 2018 at 2:58 AM Vincenzo Frascino
[off-list ref] wrote:

On 09/11/2018 16:13, Arnd Bergmann wrote:

quoted

quoted

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