K 10
svn:author
V 3
kib
K 8
svn:date
V 27
2018-05-23T21:39:29.220034Z
K 7
svn:log
V 801
x86: stop unconditionally clearing PSL_T on the trace trap.

We certainly should clear PSL_T when calling the SIGTRAP signal
handler, which is already done by all x86 sendsig(9) ABI code.  On the
other hand, there is no obvious reason why PSL_T needs to be cleared
when returning from the signal handler.  For instance, Linux allows
userspace to set PSL_T and keep tracing enabled for the desired
period.  There are userspace programs which would use PSL_T if we make
it possible, for instance sbcl.

Remember if PSL_T was set by PT_STEP or PT_SETSTEP by mean of TDB_STEP
flag, and only clear it when the flag is set.

Discussed with:	Ali Mashtizadeh
Reviewed by:	jhb (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D15054

END