Re: [RFC][PATCH] linux-2.6.2-rc2_vsyscall-gtod_B1.patch

[ebiederm _at_ xxxxxxxxxxxx (Eric W. Biederman)]

Jamie Lokier <jamie _at_ shareable.org> writes:

> Ulrich Drepper wrote:
> > ~ alternatively use the symbol table the vdso has.  Export the new code
> > only via the symbol table.  No fixed address for the function, the
> > runtime gets it from the symbol table.  glibc will use weak symbol
> > references; if the symbol isn't there, the old code is used.  This will
> > require that every single optimized syscall needs to be handled special.
> > 
> > 
> > I personally like the first approach better.  The indirection table can
> > maintained in sync with the syscall table inside the kernel.  It all
> > comes at all times from the same source.  The overhead of the memory
> > load should be neglectable.
> 
> I like the second approach more.  You can change glibc to look up the
> weak symbol for _all_ syscalls, then none of them are special and it
> will work with future kernel optimisations.

There is one more piece to consider with either approach.  The
calling conventions.

With the x86-64 optimized vsyscall the syscall number does
not need to be placed into a register, because you have used
the proper entry point.  For any syscall worth tuning in
user space I suspect that level of optimization would be
beneficial.  A fast call path that does not waste a register.

Eric
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