kern/98460: [PATCH] fpu_clean_state() cannot be disabled for not AMD processors, those are not vulnerable to FreeBSD-SA-06:14.fpu

[Bruce Evans]

On Sat, 3 Jun 2006, Rostislav Krasny wrote:

>> Description:
> When FreeBSD is running on any non AMD processor an fpu_clean_state() function
> adds unneeded operations to a context switch. My patch makes it possible
> to disable the fpu_clean_state() by rebuilding a kernel with
> "options CPU_FXSAVE_NO_LEAK".
>
> Colin Percival has nothing against my idea in general:

Hrmph.  My review implied that this should be done (not be me :-) before
committing anything.

The configuration should be dynamic and automatic, so that it doesn't
take changes to zillions of configuration files to implement and
document an option that almost no one will know to set.  I think there
is a simple feature test for the AMD misfeature.  On i386's, this
should be combined with the cpu_fxsr test so that only a single test
is needed at runtime.  On amd64's, the test would be 1 unnecessary
compare-and-branch.  I think it is not useful to have a configuration
option to avoid this compare-and-branch.

The overhead for fpu_clean_state() is a about 28 cycles.  Has anyone
actually noticed the extra context switching time for this?  It is
quite small compared with other overheads.  E.g., the one for using
the ACPI-[non]fast timecounter was about 2000 cycles at 2GHz.  Even
this was only noticeable under some loads.

Bruce