Jeff, Eli,
Am 18.03.2016 um 17:42 schrieb Jeff Dike:
> On Sat, Mar 19, 2016 at 12:13:47AM +0800, Eli Cooper wrote:
>> I agree that we should keep userspace() minimal. So what in the first
>> place is the FPU state save/restore code for? Or, what (except for
>> sigreturn) could possibly mess the FPU state of the ptrace'd process
>> without it?
>
> My (vague) recollection was that with libc in there, I couldn't be
> confident that it wouldn't unexpectedly use FP for something. So,
> saving and restoring FP state was a hedge against that happening.
>
> Jeff
>
git blame points to:
commit fbfe9c847edf57ac8232aeafb290f272289893a3
Author: Ingo van Lil <ing...@gmx.de>
Date: Wed Sep 14 16:21:23 2011 -0700
um: Save FPU registers between task switches
Some time ago Jeff prepared 42daba316557 ("uml: stop saving process FP
state") for UML to stop saving the process FP state between task
switches. The assumption was that since with SKAS0 every guest process
runs inside a host process context the host OS will take care of keeping
the proper FP state.
Unfortunately this is not true for multi-threaded applications, where
all guest threads share a single host process context yet all may use
the FPU on their own. Although I haven't verified it I suspect things
to be even worse in SKAS3 mode where all guest processes run inside a
single host process.
The patch reintroduces the saving and restoring of the FP context
between task switches.
[rich...@nod.at: Ingo posted this patch in 2009, sadly it was never applied
and got lost. Now in 2011 the problem was reported by Gunnar.]
Signed-off-by: Ingo van Lil <ing...@gmx.de>
Signed-off-by: Richard Weinberger <rich...@nod.at>
Reported-by: <gunnarlindr...@hotmail.com>
Tested-by: <gunnarlindr...@hotmail.com>
Cc: Stanislav Meduna <st...@meduna.org>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
So, it is not that easy. :=)
Thanks,
//richard
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