On Wed, May 03, 2006 at 11:44:52AM +0200, Mathieu Ropert wrote:
Hi,
I've noticed a potential bug in the spllower() implementation for Xen:
the original code saves flags through read_psl(), disable interrupts
('cli' on x86) and then even run Xspllower() (if pending interrupts) or
set new level and restore flags (with write_psl()).
On x86, saving and restoring EFLAGS includes restoring the 'IF' flag (if
it was previously set) cleared by disable_intr(), thus reenabling
interrupts.
As the 'IF' flag isn't virtualized on Xen, we may end spllower() without
reenabling interrupts.
On Xen, read_psl() and write_psl() don't change the EFLAGS register, but
read or write the evtchn_upcall_mask shared variable (see include/cpufunc.h).
So the write_psl() call will really reenable interrupts if needed. I can't
see a problem here.