Author: jhb
Date: Sat May 9 21:34:50 2020
New Revision: 360857
URL: https://svnweb.freebsd.org/changeset/base/360857
Log:
MFC 357313: 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).
Modified:
stable/12/sys/riscv/riscv/exception.S
Directory Properties:
stable/12/ (props changed)
Modified: stable/12/sys/riscv/riscv/exception.S
==============================================================================
--- stable/12/sys/riscv/riscv/exception.S Sat May 9 21:32:44 2020
(r360856)
+++ stable/12/sys/riscv/riscv/exception.S Sat May 9 21:34:50 2020
(r360857)
@@ -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)
_______________________________________________
svn-src-all@freebsd.org mailing list
https://lists.freebsd.org/mailman/listinfo/svn-src-all
To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
