K 10
svn:author
V 3
jhb
K 8
svn:date
V 27
2020-01-30T22:19:48.804237Z
K 7
svn:log
V 495
Trim duplicate CSR swaps from user exceptions.

The stack pointer is swapped with the sscratch CSR just before the
jump to cpu_exception_handler_user where the first instruction swaps
it again.  The two swaps together are a no-op, but the csr swap
instructions can be expensive (e.g. on Bluespec RISC-V cores csr swap
instructions force a full pipeline stall).

Reported by:	jrtc27
Reviewed by:	br
MFC after:	2 weeks
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D23394

END