Re: [Linux-ia64] High fpu register corruption

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Thu, 08 May 2003 16:16:13 +0200, Andreas Schwab <schwab@suse.de> said:

  Andreas> When a process clears the psr.mfh bit after using the high
  Andreas> fpu registers and then starts using them again it can
  Andreas> corrupt the fpu state of another process.  In order for
  Andreas> this to happen there must be some context switches
  Andreas> inbetween (thanks to Chris Mason for tracking this down):

Ah, _now_ it makes sense.  I got a similar bug report yesterday, but
it claimed the _old_ (2.4.19) context switch was breaking and the
new one (2.4.20) was fine.  When I looked at the old code, I couldn't
find anythign wrong with it.

  Andreas> +	} else if (ia64_get_fpu_owner() != next)			\
  Andreas> +		ia64_psr(ia64_task_regs(next))->dfh = 1;		\

I suspect what we really want to do here is something along the lines
of:

  Andreas> +	ia64_psr(ia64_task_regs(next))->dfh = (ia64_get_fpu_owner() != next);		\

This expresses the invariant we're after: the next thread has DFH set
unless it owns the FPH partition.  IIRC, this is what the UP code does
already.

	--david