On 5/5/20 10:43 AM, Alexei Starovoitov wrote:

On Sat, May 02, 2020 at 02:21:05PM -0500, Josh Poimboeuf wrote:

quoted

Ideally we would get rid of that label and just change all the 'goto
select_insn' to 'goto *jumptable[insn->code]'.  That allows objtool to
follow the code in both retpoline and non-retpoline cases.  It also
simplifies the code flow and (IMO) makes it easier for GCC to find
optimizations.

No. It's the opposite. It's not simplifying the code. It pessimizes
compilers.

quoted

However, for the RETPOLINE=y case, that simplification actually would
cause GCC to grow the function text size by 40%.  

It pessimizes and causes text increase, since the version of gcc
you're testing with cannot combine indirect gotos back into direct.

quoted

I thought we were in
agreement that significant text growth would be universally bad,
presumably because of i-cache locality/pressure issues.  

No. As I explained before the extra code could give performance
increase depending on how branch predictor is designed in HW.

quoted

Or, if you want to minimize the patch's impact on other arches, and keep
the current patch the way it is (with bug fixed and changed patch
description), that's fine too.  I can change the patch description
accordingly.

Or if you want me to measure the performance impact of the +40% code
growth, and *then* decide what to do, that's also fine.  But you'd need
to tell me what tests to run.

I'd like to minimize the risk and avoid code churn,
so how about we step back and debug it first?
Which version of gcc are you using and what .config?
I've tried:
Linux version 5.7.0-rc2 (gcc version 10.0.1 20200505 (prerelease) (GCC)
CONFIG_UNWINDER_ORC=y
# CONFIG_RETPOLINE is not set

and objtool didn't complain.
I would like to reproduce it first before making any changes.

Also since objtool cannot follow the optimizations compiler is doing
how about admit the design failure and teach objtool to build ORC
(and whatever else it needs to build) based on dwarf for the functions where
it cannot understand the assembly code ?
Otherwise objtool will forever be playing whackamole with compilers.

Hi,

I see the objtool warning:
kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x33: call without frame pointer save/setup

when using:
gcc (SUSE Linux) 9.3.1 20200406 [revision 6db837a5288ee3ca5ec504fbd5a765817e556ac2]

with the attached config file.

thanks.
-- 
~Randy