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Trim duplicate CSR swaps from user exceptions.

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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
This commit is contained in:
John Baldwin 2020-01-30 22:19:48 +00:00
parent 3ff65f71cb
commit 5a02cd314d
1 changed files with 0 additions and 2 deletions
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2
sys/riscv/riscv/exception.S
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@ -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)