Date: Tue, 1 Jun 2004 23:48:46 -0700 From: David Schultz <das@FreeBSD.ORG> To: amd64@FreeBSD.ORG Subject: Initial FP exception flags incorrect on amd64 Message-ID: <20040602064846.GA6124@VARK.homeunix.com>
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I discovered that new processes on amd64 have the inexact flag
raised by default, at least on sledge. However, all the sticky
flags should be clear initially. Here is a program that
demonstrates the problem:
#include <stdio.h>
int
main(int argc, char *argv[])
{
int r;
__asm("stmxcsr %0" : "=m" (r));
printf("got 0x%02x, expecting 0x00\n", r & 0x3f);
}
I don't have any amd64 hardware of my own to test kernel patches
on, but if I were to make a wild guess as to how to solve the
problem, it would be the following patch. I would appreciate it
if someone could address the problem, or at least let me know
whether my proposed fix works.
Index: sys/amd64/amd64/fpu.c
===================================================================
RCS file: /cvs/src/sys/amd64/amd64/fpu.c,v
retrieving revision 1.149
diff -u -r1.149 fpu.c
--- fpu.c 5 Apr 2004 21:25:51 -0000 1.149
+++ fpu.c 2 Jun 2004 06:08:34 -0000
@@ -73,6 +73,7 @@
#define fnstsw(addr) __asm __volatile("fnstsw %0" : "=m" (*(addr)))
#define fxrstor(addr) __asm("fxrstor %0" : : "m" (*(addr)))
#define fxsave(addr) __asm __volatile("fxsave %0" : "=m" (*(addr)))
+#define stmxcsr(addr) __asm("stmxcsr %0" : "=m" (*(addr)))
#define start_emulating() __asm("smsw %%ax; orb %0,%%al; lmsw %%ax" \
: : "n" (CR0_TS) : "ax")
#define stop_emulating() __asm("clts")
@@ -119,6 +120,8 @@
fninit();
control = __INITIAL_FPUCW__;
fldcw(&control);
+ control = __INITIAL_MXCSR__;
+ stmxcsr(&control);
fxsave(&fpu_cleanstate);
start_emulating();
fpu_cleanstate_ready = 1;
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