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)
