From: Zachary Amsden 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 Cc: Andi Kleen Cc: Jeremy Fitzhardinge Cc: Rusty Russell Cc: Chris Wright Signed-off-by: Andrew Morton --- arch/i386/kernel/process.c | 9 +++++++++ 1 files changed, 9 insertions(+) diff -puN arch/i386/kernel/process.c~paravirt-iopl-handling-for-paravirt-guests arch/i386/kernel/process.c --- a/arch/i386/kernel/process.c~paravirt-iopl-handling-for-paravirt-guests +++ a/arch/i386/kernel/process.c @@ -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) _