runtime, syscall: use SYSCALL instruction on FreeBSD.

This manually reverts 555da73c from #6372 which implies a
minimum FreeBSD version of 8-STABLE.
Updates docs to mention new minimum requirement.

Fixes #9627

Change-Id: I40ae64be3682d79dd55024e32581e3e5e2be8aa7
Reviewed-on: https://go-review.googlesource.com/3020Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install.html

@@ -14,9 +14,10 @@
 
 <p>
 <a href="https://golang.org/dl/" target="_blank">Official binary
-distributions</a> are available for the FreeBSD (release 8 and above), Linux, Mac OS X (Snow Leopard
-and above), and Windows operating systems and the 32-bit (<code>386</code>) and
-64-bit (<code>amd64</code>) x86 processor architectures.
+distributions</a> are available for the FreeBSD (release 8-STABLE and above),
+Linux, Mac OS X (Snow Leopard and above), and Windows operating systems and
+the 32-bit (<code>386</code>) and 64-bit (<code>amd64</code>) x86 processor
+architectures.
 </p>
 
 <p>
@@ -44,7 +45,7 @@ proceeding. If your OS or architecture is not on the list, it's possible that
 <th align="center">Notes</th>
 </tr>
 <tr><td colspan="3"><hr></td></tr>
-<tr><td>FreeBSD 8 or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>
+<tr><td>FreeBSD 8-STABLE or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>
 <tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported; no binary distribution for ARM yet</td></tr>
 <tr><td>Mac OS X 10.6 or later</td> <td>amd64, 386</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
 <tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>

src/runtime/sys_freebsd_amd64.s

@@ -10,31 +10,6 @@
 #include "go_tls.h"
 #include "textflag.h"
 
-// FreeBSD 8, FreeBSD 9, and older versions that I have checked
-// do not restore R10 on exit from a "restarted" system call
-// if you use the SYSCALL instruction. This means that, for example,
-// if a signal arrives while the wait4 system call is executing,
-// the wait4 internally returns ERESTART, which makes the kernel
-// back up the PC to execute the SYSCALL instruction a second time.
-// However, since the kernel does not restore R10, the fourth
-// argument to the system call has been lost. (FreeBSD 9 also fails
-// to restore the fifth and sixth arguments, R8 and R9, although
-// some earlier versions did restore those correctly.)
-// The broken code is in fast_syscall in FreeBSD's amd64/amd64/exception.S.
-// It restores only DI, SI, DX, AX, and RFLAGS on system call return.
-// http://fxr.watson.org/fxr/source/amd64/amd64/exception.S?v=FREEBSD91#L399
-//
-// The INT $0x80 system call path (int0x80_syscall in FreeBSD's 
-// amd64/ia32/ia32_exception.S) does not have this problem,
-// but it expects the third argument in R10. Instead of rewriting
-// all the assembly in this file, #define SYSCALL to a safe simulation
-// using INT $0x80.
-//
-// INT $0x80 is a little slower than SYSCALL, but correctness wins.
-//
-// See golang.org/issue/6372.
-#define SYSCALL MOVQ R10, CX; INT $0x80
-	
 TEXT runtime·sys_umtx_op(SB),NOSPLIT,$0
 	MOVQ addr+0(FP), DI
 	MOVL mode+8(FP), SI

src/syscall/asm_freebsd_amd64.s

@@ -12,11 +12,6 @@
 // System call support for AMD64, FreeBSD
 //
 
-// The SYSCALL variant for invoking system calls is broken in FreeBSD.
-// See comment at top of ../runtime/sys_freebsd_amd64.c and
-// golang.org/issue/6372.
-#define SYSCALL MOVQ R10, CX; INT $0x80
-
 // func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
 // func Syscall6(trap int64, a1, a2, a3, a4, a5, a6 int64) (r1, r2, err int64);
 // func Syscall9(trap int64, a1, a2, a3, a4, a5, a6, a7, a8, a9 int64) (r1, r2, err int64)