[PATCH] x86: Require full ISA support for x86-64 level marker [BZ #27318]

[Joseph Myers]

On Tue, 2 Feb 2021, H.J. Lu wrote:

> On Tue, Feb 2, 2021 at 3:11 PM Joseph Myers <joseph@codesourcery.com> wrote:
> >
> > On Tue, 2 Feb 2021, H.J. Lu via Libc-alpha wrote:
> >
> > > Instead, we should require full ISA support for x86-64 level marker to
> > > detect such case:
> > >
> > > In file included from ../sysdeps/x86/abi-note.c:28:
> > > ../sysdeps/x86/isa-level.c:62:5: error: #error "Invalid ISAs for x86-64 ISA level v3"
> > >    62 | #   error "Invalid ISAs for x86-64 ISA level v3"
> > >       |     ^~~~~
> >
> > When does this error occur (what conditions for compilation /
> > configuration of glibc)?
> 
> It happens at compile time when glibc is built with "-march=sandybridge".

That's bad.  Since glibc supports execution on Sandy Bridge processors, 
compilation with -march=sandybridge should (a) work, with no special 
configure options needed and (b) produce a glibc that works on Sandy 
Bridge, with no special configure options needed.  I understand that bug 
27318 is reporting that (b) fails at present.  We need to fix (b) without 
breaking (a).

This is not specific at all to x86_64.  It applies to all architectures 
and processors supported by glibc: compiling with a compiler that defaults 
to any such processor should just work, regardless of how that processor 
relates to particular ISA levels in the glibc-hwcaps machinery.

> We can add a configure option, --disable-isa-level, to unset
> INCLUDE_X86_ISA_LEVEL.  The resulting libc.so doesn't have a marker
> and won't run on all machines.

No special configure option should be needed for (a) and (b) to hold.  
They are general principles for any processor supported by glibc, for any 
architecture.

-- 
Joseph S. Myers
joseph@codesourcery.com