i386 Go programs crash on amd64

[Konstantin Belousov]

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
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