[PATCH] i386: iOPL handling for paravirt guests

I found a clever way to make the extra IOPL switching invisible to
non-paravirt compiles - since kernel_rpl is statically defined to be zero
there, and only non-zero rpl kernel have a problem restoring IOPL, as popf
does not restore IOPL flags unless run at CPL-0.
Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>

arch/i386/kernel/process.c

@@ -669,6 +669,15 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
 	 */
 	load_TLS(next, cpu);
 
+	/*
+	 * Restore IOPL if needed.  In normal use, the flags restore
+	 * in the switch assembly will handle this.  But if the kernel
+	 * is running virtualized at a non-zero CPL, the popf will
+	 * not restore flags, so it must be done in a separate step.
+	 */
+	if (get_kernel_rpl() && unlikely(prev->iopl != next->iopl))
+		set_iopl_mask(next->iopl);
+
 	/*
 	 * Now maybe handle debug registers and/or IO bitmaps
 	 */