On 04/06/15 07:38, H. Peter Anvin wrote: > On 06/03/2015 02:31 AM, Andrew Cooper wrote: >> There appears to be no formal statement of what pv_irq_ops.save_fl() is >> supposed to return precisely. Native returns the full flags, while lguest and >> Xen only return the Interrupt Flag, and both have comments by the >> implementations stating that only the Interrupt Flag is looked at. This may >> have been true when initially implemented, but no longer is. >> >> To make matters worse, the Xen PVOP leaves the upper bits undefined, making >> the BUG_ON() undefined behaviour. Experimentally, this now trips for 32bit PV >> guests on Broadwell hardware. The BUG_ON() is consistent for an individual >> build, but not consistent for all builds. It has also been a sitting timebomb >> since SMAP support was introduced. >> >> Use native_save_fl() instead, which will obtain an accurate view of the AC >> flag. > Could we fix the Xen pvops wrapper instead to not do things like this? > > -hpa > > We could, and I have a patch for that, but the check would still then be broken in lguest, and it makes a hotpath rather longer. Either pv_irq_ops.save_fl() gets defined to handle all flags, and Xen & lguest need correcting in this regard, or save_fl() gets defined to handle the interrupt flag only, and this becomes the single problematic caller in the codebase. The problem with expanding save_fl() to handle all flags is that restore_fl() should follow suit, and there are a number of system flags are inapplicable in such a situation. ~Andrew -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html