svn commit: r357313 - head/sys/riscv/riscv

[John Baldwin]

Author: jhb
Date: Thu Jan 30 22:19:48 2020
New Revision: 357313
URL: https://svnweb.freebsd.org/changeset/base/357313

Log:
  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

Modified:
  head/sys/riscv/riscv/exception.S

Modified: head/sys/riscv/riscv/exception.S
==============================================================================
--- head/sys/riscv/riscv/exception.S	Thu Jan 30 20:05:05 2020	(r357312)
+++ head/sys/riscv/riscv/exception.S	Thu Jan 30 22:19:48 2020	(r357313)
@@ -208,7 +208,6 @@ ENTRY(cpu_exception_handler)
 	csrrw	sp, sscratch, sp
 	beqz	sp, 1f
 	/* User mode detected */
-	csrrw	sp, sscratch, sp
 	j	cpu_exception_handler_user
 1:
 	/* Supervisor mode detected */
@@ -225,7 +224,6 @@ ENTRY(cpu_exception_handler_supervisor)
 END(cpu_exception_handler_supervisor)
 
 ENTRY(cpu_exception_handler_user)
-	csrrw	sp, sscratch, sp
 	save_registers 0
 	mv	a0, sp
 	call	_C_LABEL(do_trap_user)