[PATCH 3/6] IOPL handling for paravirt guests

[Zachary Amsden <zach@vmware.com>]

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.

Subject: IOPL handling for paravirt guests
Signed-off-by: Zachary Amsden <zach@vmware.com>

diff -r 8110943fd7ad arch/i386/kernel/process.c
--- a/arch/i386/kernel/process.c	Thu Dec 14 16:15:20 2006 -0800
+++ b/arch/i386/kernel/process.c	Thu Dec 14 16:21:57 2006 -0800
@@ -665,6 +665,15 @@ struct task_struct fastcall * __switch_t
 	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
 	 */
 	if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW)