Author: kib
Date: Sat Jul 21 21:49:05 2012
New Revision: 238677
URL: http://svn.freebsd.org/changeset/base/238677
Log:
MFamd64 r238668:
Stop clearing x87 exceptions in the #MF handler.
Requested by: bde
MFC after: 1 week
Modified:
head/sys/i386/isa/npx.c
Modified: head/sys/i386/isa/npx.c
==============================================================================
--- head/sys/i386/isa/npx.c Sat Jul 21 21:39:23 2012 (r238676)
+++ head/sys/i386/isa/npx.c Sat Jul 21 21:49:05 2012 (r238677)
@@ -583,21 +583,21 @@ static char fpetable[128] = {
};
/*
- * Preserve the FP status word, clear FP exceptions, then generate a SIGFPE.
+ * Read the FP status and control words, then generate si_code value
+ * for SIGFPE. The error code chosen will be one of the
+ * FPE_... macros. It will be sent as the second argument to old
+ * BSD-style signal handlers and as "siginfo_t->si_code" (second
+ * argument) to SA_SIGINFO signal handlers.
*
- * Clearing exceptions is necessary mainly to avoid IRQ13 bugs. We now
- * depend on longjmp() restoring a usable state. Restoring the state
- * or examining it might fail if we didn't clear exceptions.
+ * Some time ago, we cleared the x87 exceptions with FNCLEX there.
+ * Clearing exceptions was necessary mainly to avoid IRQ13 bugs. The
+ * usermode code which understands the FPU hardware enough to enable
+ * the exceptions, can also handle clearing the exception state in the
+ * handler. The only consequence of not clearing the exception is the
+ * rethrow of the SIGFPE on return from the signal handler and
+ * reexecution of the corresponding instruction.
*
- * The error code chosen will be one of the FPE_... macros. It will be
- * sent as the second argument to old BSD-style signal handlers and as
- * "siginfo_t->si_code" (second argument) to SA_SIGINFO signal handlers.
- *
- * XXX the FP state is not preserved across signal handlers. So signal
- * handlers cannot afford to do FP unless they preserve the state or
- * longjmp() out. Both preserving the state and longjmp()ing may be
- * destroyed by IRQ13 bugs. Clearing FP exceptions is not an acceptable
- * solution for signals other than SIGFPE.
+ * For XMM traps, the exceptions were never cleared.
*/
int
npxtrap()
@@ -623,9 +623,6 @@ npxtrap()
fnstcw(&control);
fnstsw(&status);
}
-
- if (PCPU_GET(fpcurthread) == curthread)
- fnclex();
critical_exit();
return (fpetable[status & ((~control & 0x3f) | 0x40)]);
}
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