Re: [PATCH] i386/x86_64 fpu: fix x87 tag word simulation using fxsave

From: Linus Torvalds (torvalds_at_osdl.org)
Date: 01/17/05

  • Next message: Chris Caputo: "Re: 3ware driver (3w-xxxx) in 2.6.10: procfs entry" 
    Date:	Sun, 16 Jan 2005 19:48:42 -0800 (PST)
To: Roland McGrath <roland@redhat.com>

    On Sun, 16 Jan 2005, Roland McGrath wrote:
    >
    > Note, this code is identical in 2.4 so this fix applies there as well.

    Uhh.. "(i + 8 - tos) & 7" makes no sense.

    It's not only mathematically the same as "(i - tos) & 7", but I don't even
    see how you got to the "+ 8" in the first place..

    Also, we're not using C++, so we don't do the "const" part of

            const unsigned int tos = ...

    thing. So suggested version with trivial fixes appended (technically, you
    don't need the "& 7" on the tos calculation either, but hey, at least that
    one I see why it's there, and agree with - the compiler may well be able
    to optimize both of these things away, so keeping the source in its
    "logical" format makes sense). Does this still work for you?

    > Fortunately, only the manuals differ and all the chips actually agree.

    Hmm.

    What happens if it was in MMX mode? If I remember correctly, different
    CPU's do different things for MMX (some rotate them so that "mmx0" is
    always the same as "st(0)", some don't, and/or have separate hw registers
    altogether for it)? I realize we don't probably care, I'm just wondering
    if somebody knows...

                            Linus 

    ----
===== arch/i386/kernel/i387.c 1.22 vs edited =====
--- 1.22/arch/i386/kernel/i387.c	2004-10-28 00:39:55 -07:00
+++ edited/arch/i386/kernel/i387.c	2005-01-16 19:38:28 -08:00
@@ -111,16 +111,17 @@
 static inline unsigned long twd_fxsr_to_i387( struct i387_fxsave_struct *fxsave )
 {
 	struct _fpxreg *st = NULL;
+	unsigned int tos = (fxsave->swd >> 11) & 7;
 	unsigned long twd = (unsigned long) fxsave->twd;
 	unsigned long tag;
 	unsigned long ret = 0xffff0000u;
 	int i;
 
-#define FPREG_ADDR(f, n)	((char *)&(f)->st_space + (n) * 16);
+#define FPREG_ADDR(f, n)	((void *)&(f)->st_space + (n) * 16);
 
 	for ( i = 0 ; i < 8 ; i++ ) {
 		if ( twd & 0x1 ) {
-			st = (struct _fpxreg *) FPREG_ADDR( fxsave, i );
+			st = FPREG_ADDR( fxsave, (i - tos) & 7 );
 
 			switch ( st->exponent & 0x7fff ) {
 			case 0x7fff:
===== arch/x86_64/ia32/fpu32.c 1.8 vs edited =====
--- 1.8/arch/x86_64/ia32/fpu32.c	2004-05-30 12:49:35 -07:00
+++ edited/arch/x86_64/ia32/fpu32.c	2005-01-16 19:39:20 -08:00
@@ -28,16 +28,17 @@
 static inline unsigned long twd_fxsr_to_i387(struct i387_fxsave_struct *fxsave)
 {
 	struct _fpxreg *st = NULL;
+	unsigned long tos = (fxsave->swd >> 11) & 7;
 	unsigned long twd = (unsigned long) fxsave->twd;
 	unsigned long tag;
 	unsigned long ret = 0xffff0000;
 	int i;
 
-#define FPREG_ADDR(f, n)	((char *)&(f)->st_space + (n) * 16);
+#define FPREG_ADDR(f, n)	((void *)&(f)->st_space + (n) * 16);
 
 	for (i = 0 ; i < 8 ; i++) {
 		if (twd & 0x1) {
-			st = (struct _fpxreg *) FPREG_ADDR( fxsave, i );
+			st = FPREG_ADDR( fxsave, (i - tos) & 7 );
 
 			switch (st->exponent & 0x7fff) {
 			case 0x7fff:
-
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