[PATCH v2] x86: Require full ISA support for x86-64 level marker [BZ #27318]

[H.J. Lu]

On Wed, Feb 3, 2021 at 6:14 AM Joseph Myers <joseph@codesourcery.com> wrote:
>
> 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.

Here is the updated patch to disable x86-64 level marker for
-march=sandybridge which enables ISAs between v2 and v3.

-- 
H.J.
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