Author: mjg
Date: Thu Sep 20 13:29:43 2018
New Revision: 338828
URL: https://svnweb.freebsd.org/changeset/base/338828
Log:
amd64: move fusufault after all users
A lot of function have the following check:
cmpq %rax,%rdi /* verify address is valid */
ja fusufault
The label is present earlier in kernel .text, which means this is a jump
backwards. Absent any information in branch predictor, the cpu predicts it
as taken. Since it is almost never taken in practice, this results in a
completely avoidable misprediction.
Move it past all consumers, so that it is predicted as not taken.
Approved by: re (kib)
Modified:
head/sys/amd64/amd64/support.S
Modified: head/sys/amd64/amd64/support.S
==============================================================================
--- head/sys/amd64/amd64/support.S Thu Sep 20 11:44:36 2018
(r338827)
+++ head/sys/amd64/amd64/support.S Thu Sep 20 13:29:43 2018
(r338828)
@@ -755,16 +755,6 @@ ENTRY(fubyte_smap)
ret
END(fubyte_smap)
- ALIGN_TEXT
- /* Fault entry clears PSL.AC */
-fusufault:
- movq PCPU(CURPCB),%rcx
- xorl %eax,%eax
- movq %rax,PCB_ONFAULT(%rcx)
- decq %rax
- POP_FRAME_POINTER
- ret
-
/*
* Store a 64-bit word, a 32-bit word, a 16-bit word, or an 8-bit byte to
* user memory.
@@ -915,6 +905,16 @@ ENTRY(subyte_smap)
POP_FRAME_POINTER
ret
END(subyte_smap)
+
+ ALIGN_TEXT
+ /* Fault entry clears PSL.AC */
+fusufault:
+ movq PCPU(CURPCB),%rcx
+ xorl %eax,%eax
+ movq %rax,PCB_ONFAULT(%rcx)
+ decq %rax
+ POP_FRAME_POINTER
+ ret
/*
* copyinstr(from, to, maxlen, int *lencopied)
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