Date: Sun, 4 Jun 2006 15:40:33 GMT From: Rostislav Krasny <rosti.bsd@gmail.com> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/98460 : [kernel] [patch] fpu_clean_state() cannot be disabled for not AMD processors, those are not vulnerable to FreeBSD-SA-06:14.fpu Message-ID: <200606041540.k54FeX5v035825@freefall.freebsd.org>
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The following reply was made to PR kern/98460; it has been noted by GNATS. From: Rostislav Krasny <rosti.bsd@gmail.com> To: Bruce Evans <bde@zeta.org.au> Cc: bug-followup@FreeBSD.org Subject: Re: kern/98460 : [kernel] [patch] fpu_clean_state() cannot be disabled for not AMD processors, those are not vulnerable to FreeBSD-SA-06:14.fpu Date: Sun, 4 Jun 2006 18:36:23 +0300 On Sun, 4 Jun 2006, Bruce Evans 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. David Xu had proposed something like that. But from Colin Percival's reply I understood that it is hard to be done effectively. See their discussion by the first URL in this PR. > 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. Future amd64 processors may be changed back, to the classical behavior of FXSAVE and FXRSTOR instructions.
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