Re: [dpdk-dev] [PATCH] build: check functionality rather than binutils version

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jun 17, 2020 at 01:45:57PM +0200, Thomas Monjalon wrote:
> 17/06/2020 12:40, Bruce Richardson:
> > Rather than checking the binutils version number, which can lead to
> > unnecessary disabling of AVX512 if fixes have been backported to distro
> > versions, we can instead check the output of "as" from binutils to see if
> > it is correct.
> > 
> > These checks use the minimal assembly reproduction code posted to the
> > public bug tracker for gcc/binutils for those issues [1][2]. If the
> > binutils bug is present, the instruction parameters - specifically the
> > displacement parameter - will be different in the disassembled output
> > compared to the input. Therefore each check involves assembling a single
> > instruction and disassembling it again, checking that the two match.
> 
> Whaoh, that's a very specific optimization :)
> Which distro is affected (backported binutils fix)?
> 

Not an optimization, it's a bug, which necessitates disabling a whole
instruction set, so we should really check for the bug rather than assuming
all versions are affected. From [2] below, we have confirmation that the
fix has been backported to both 2.30 and 2.31 binutils branches.

> > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90028
> > [2] https://sourceware.org/bugzilla/show_bug.cgi?id=23465
> > 
> > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > ---
> > --- a/buildtools/meson.build
> > +++ b/buildtools/meson.build
> > -subdir('pmdinfogen')
> 
> Why removing pmdinfogen?

The pmdinfogen requires values that are set in the config folder, but the
config folder processing now uses scripts that are in the buildtools
folder. While we could refer to them using relative paths, I felt it better
to separate out the compilation of pmdinfogen to the rest of the buildutils
processing, which is simply assigning variables to scripts to make them
easier to call later in the processing.

> 
> > --- a/meson.build
> > +++ b/meson.build
> > +# do configuration and get tool paths
> > +subdir('buildtools')
> >  subdir('config')
> >  
> >  # build libs and drivers
> > -subdir('buildtools')
> > +subdir('buildtools/pmdinfogen')
> 
> Is it related to binutils check?
> 
Pretty much. It just makes it easier to have the config processing use
scripts in the buildtools folder.

> [...]
> > -	ldver = run_command('ld', '-v').stdout().strip()
> > -	if ldver.contains('2.30') and cc.has_argument('-mno-avx512f')
> > +	as_ok = run_command(binutils_avx512_check)
> > +	if as_ok.returncode() != 0 and cc.has_argument('-mno-avx512f')
> 
> "as_ok" is difficult to understand.
> I would suggest "binutils_avx512_ok".
> 
Sure, can change for v2 - though I'd suggest the slightly shorter
binutils_ok.

> > +		message('Binutils error with AVX512 assembly, disabling AVX512 support')
> 
> It looks like something which should be part of meson itself.
> 
That would certainly be nice to have, but I consider it infeasible, since
there will be, even in the best case, a lag between a bug being discovered
and workarounds making its way into tools. Even then you still have an
update problem to work around it - update binutils and bug goes away, or
update meson and you can work around it. In both cases you are requiring
the user to update something, so fixing in DPDK is best.

/Bruce