Andy Lutomirski <[email protected]> writes: > On 04/20/2015 10:09 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.
That should work for lguest. Indeed, it does (in practice those bits are 0). Tested-by: Rusty Russell <[email protected]> (lguest) Thanks, Rusty. >> Signed-off-by: Andrew Cooper <[email protected]> >> CC: Thomas Gleixner <[email protected]> >> CC: Ingo Molnar <[email protected]> >> CC: H. Peter Anvin <[email protected]> >> CC: [email protected] >> CC: [email protected] >> CC: Konrad Rzeszutek Wilk <[email protected]> >> CC: Boris Ostrovsky <[email protected]> >> CC: David Vrabel <[email protected]> >> CC: xen-devel <[email protected]> >> CC: Rusty Russell <[email protected]> >> CC: [email protected] >> >> --- >> This patch is RFC because I am not certain that native_save_fl() is >> necessarily the correct solution on lguest, but it does seem that >> setup_smap() >> wants to check the actual AC bit, rather than an idealised value. >> >> A different approach, given the dual nature of the AC flag now is to gate >> setup_smap() on a kernel rpl of 0. SMAP necessarily can't be used in a >> paravirtual situation where the kernel runs in cpl > 0. >> >> Another different approach would be to formally state that >> pv_irq_ops.save_fl() needs to return all the flags, which would make >> local_irq_save() safe to use in this circumstance, but that makes a hotpath >> longer for the sake of a single boot time check. > > ...which reminds me: > > Why does native_restore_fl restore anything other than IF? A branch and > sti should be considerably faster than popf. > > Also, if we did this, could Xen use PVI and then use native_restore_fl > and avoid lots of pvops? > > --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
