Re: [PATCH] Updates to FXSR/SSE support in 2.4.0-test1

[Mark Kettenis <kettenis at wins dot uva dot nl>]

   Date: Sat, 10 Jun 2000 12:51:46 -0600
   From: Gareth Hughes <gareth@precisioninsight.com>

   Here are the patches against 2.4.0-test1-ac12 for the fixes to the PIII
   SSE/FXSR support.

Sorry, but I don't think this is right:

 #define save_fpu(tsk) do { \
-	asm volatile("fxsave %0 ; fwait" \
-		     : "=m" (tsk->thread.i387.hard)); \
+	asm volatile("fnstenv %0 ; fxsave %1 ; fwait" \
+		     : "=m" (tsk->thread.i387.hard), \
+		       "=m" (tsk->thread.i387.hard.fxsr_space[0])); \

If I'm not mistaken this means that you've added an additional
overhead on *every* task switch between FPU-using tasks.  Instead you
should only convert to the old fsave format when that's actually
needed:

 1. When setting up a signal context for invoking a signal handler.

 2. For the PTRACE_GETFPREGS request.

I also don't understand why you expose the strange user_xfpregs_struct
format via the PTRACE_{GET,SET}XFPREGS requests.  Why don't you just
provide the 512-byte FXSAVE area?

All in all, I think that the origional code in 2.4.0test1 is much
closer to what the endresult should look like.  Just add the
PTRACE_{GET,SET}XFPREGS requests that return, write the FXSAVE data
(should be *very* simple), add the necessary bits for setting up the
signal frame (more or less what's in your patch, but this is the place
where the FXSAVE -> FSAVE conversion should be done), and add some
code to the core dumping code to add an extra note with the FXSAVE
area.

Mark