On Thu, May 29, 2014 at 08:55:42AM +1000, Peter Jeremy wrote: > As I wrote in my initial mail, I am not certain whether this is a problem > with Go or FreeBSD. And having done some poking at corefiles with gdb > (you need gdb7.6 from ports to grok the Go debug information), I have > found that all my programs are dying at: > mov %gs:0xfffffff8,%ecx > but it seems to be timing related as to when the offending instruction > is executed. I will do some further investigation into how segment > wraparound is handled.
Hm, I think I know what is going on. Try this, please.
At least, your test binary worked for me.
diff --git a/sys/amd64/amd64/exception.S b/sys/amd64/amd64/exception.S
index d4e58b1..d51078d 100644
--- a/sys/amd64/amd64/exception.S
+++ b/sys/amd64/amd64/exception.S
@@ -721,21 +721,38 @@ ld_fsbase:
pushfq
cli
movl $MSR_GSBASE,%ecx
+ /* Save current kernel %gs base into %r12d:%r13d */
rdmsr
+ movl %eax,%r12d
+ movl %edx,%r13d
.globl ld_gs
ld_gs:
movw %si,%gs
+ /* Save user %gs base into %r14d:%r15d */
+ rdmsr
+ movl %eax,%r14d
+ movl %edx,%r15d
+ /* Restore kernel %gs base */
+ movl %r12d,%eax
+ movl %r13d,%edx
wrmsr
popfq
+ /*
+ * Restore user %gs base, either from PCB if used for TLS, or
+ * from the previously saved msr read.
+ */
+ movl $MSR_KGSBASE,%ecx
cmpw $KUG32SEL,%si
jne 1f
- movl $MSR_KGSBASE,%ecx
movl PCB_GSBASE(%r8),%eax
movl PCB_GSBASE+4(%r8),%edx
+ jmp ld_gsbase
+1:
+ movl %r14d,%eax
+ movl %r15d,%edx
.globl ld_gsbase
ld_gsbase:
- wrmsr
-1:
+ wrmsr /* May trap if non-canonical, but only for TLS. */
.globl ld_es
ld_es:
movw TF_ES(%rsp),%es
pgp3LpzNAPYhQ.pgp
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