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Subject: Re: MULX on haswell triggers ILLOP
References: <86wql4onvy.fsf@shell.gmplib.org>
From: Torbjorn Granlund <tg@gmplib.org>
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Date: Wed, 23 Oct 2013 19:42:46 +0200
In-Reply-To: <86wql4onvy.fsf@shell.gmplib.org> (Torbjorn Granlund's message of
 "Wed\, 23 Oct 2013 02\:02\:25 +0200")
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Torbjorn Granlund <tg@gmplib.org> writes:

  I have an Intel Haswell system which runs Xen/NetBSD with Debian 7.2,
  NetBSD 6.1.2 and FreeBSD 9.2 guests.
=20=20
  The FreeBSD guest casts an ILLOP when fed with the new MULX instruction.
  This is a plain integer register based instruction which should require
  zero kernel support.
=20=20
  MULX is part of the BMI2 extensions.
=20=20
  The instruction works fine in the other guest systems.
=20=20
  What is going on?  How can FreeBSD reject this instruction?
=20=20
  I have not tried to run FreeBSD on the bare metal, so I cannot tell if
  BMI2 works as expected there.

It turns out to be another strike of an FreeBSD m4 eval bug.

http://www.freebsd.org/cgi/query-pr.cgi?pr=3D166994

Unfortunately, this means that GMP will not work on Haswell CPUs running
FreeBSD, where we assemble unsupported instructions using m4 macros.
That is a workaround for FreeBSD's seriously outdated binutils.  Of
course, we could implement a workaround for the m4 bug, but that
workaround would be more complex than a bugfix for m4.

My hopes that FreeBSD will fix this bug are not high, given that it has
been known for 1.5 years.

--=20
Torbj=C3=B6rn