Ping?  I haven't heard anything on this.

Thanks,

-corey

On 09/18/2014 07:58 AM, Corey Minyard wrote:

On 09/18/2014 04:58 AM, Ralf Baechle wrote:

quoted

On Tue, Sep 16, 2014 at 04:45:25PM -0500, minyard@acm.org wrote:

quoted

From: Corey Minyard <redacted>

The MIPS frame save code was just saving a few registers, enough to
do a backtrace if every function set up a frame.  However, this is
not working if you are using DWARF unwinding, because most of the
registers are wrong.  This was causing kdump backtraces to be short
or bogus.

So save all the registers.

The stratey of partial and full stack frames was developed in '97 to bring
down the syscall overhead.  It certaily was very effective - it brought
down the syscall latency to the level of Alphas running at much higher
clock.

That certainly worked well back then for kernel 2.0 / 2.2.  But the syscall
code has become much more complex.  Since then support for 64 bit kernels,
two 32 bit ABIs running on a 64 bit kernels and numerous features that
changed the once simple syscall path have been implemented.  My gut feeling
is it might be worth to yank out the whole optimization to see how much
code complexity we get rid of in exchange for how much extra syscall
latency.

I"m not sure I understand.  From what I can tell, this code is only
called by
things that print stack traces, kdb, and kexec/kdump.  So it shouldn't be in
any normal syscall path.

This patch will currently only help kdump, but it will be necessary if
anyone
adds MIPS support for DWARF unwinding for stack traces.  And you'd have
to fix some things in context switching, too, I think.

From what I can tell the partial save for syscalls is a good idea.  You
don't have
to save half the registers and it doesn't affect tracebacks, kdump, or
anything else
like that.

-corey