On Wed, Apr 27, 2011 at 10:47:04AM +0300, Avi Kivity wrote: > On 04/26/2011 06:55 PM, Paul E. McKenney wrote: > >On Tue, Apr 26, 2011 at 03:38:24PM +0300, Gleb Natapov wrote: > >> Hello Paul, > >> > >> I have a question about RCU + KVM. KVM does not hold any references to RCU > >> protected data when it switches CPU into a guest mode. In fact switching > >> to a guest mode is very similar to exiting to userspase from RCU point > >> of view. In addition CPU may stay in a guest mode for quite a long time > >> (up to one time slice). It looks like it will be beneficial to treat guest > >> mode as quiescent state, just like user-mode execution. How can this be > >> done? I was trying to find how RCU knows about cpu entering user-mode, > >> but it seems that it does this by checking CPU mode in a timer interrupt > >> (update_process_times()->rcu_check_callbacks()). This will not work for > >> guest mode detection since timer interrupt will kick CPU out of a guest > >> mode and timer interrupt will always see CPU in kernel mode. Do we have > >> a simple function to call to notify RCU that CPU passed quiescent state > >> which we can call just before entering guest? > > > >Hello, Gleb, > > > >You could call rcu_note_context_switch(), passing it the current > >CPU. Please note that preemption -must- be disabled when calling > >this. You could call this just after exiting the guest as well > >as just before entering guest. > > It's expected that after exiting, we'd spend a very short time in > the kernel, and then either re-enter the guest, exit to userspace, > or switch to another task. So I think calling it just before entry > should be sufficient. Agreed, sorry for my confusion! > Looking at the code, I see rcu_note_context_switch() calls > rcu_sched_qs(), which does > > rdp->passed_quiesc_completed = rdp->gpnum - 1; > barrier(); > rdp->passed_quiesc = 1; > > and also calls rcu_preempt_note_context_switch(), which calls > rcu_preempt_qs(), which does > > rdp->passed_quiesc_completed = rdp->gpnum - 1; > barrier(); > rdp->passed_quiesc = 1; > current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS; > > the similarity is remarkable. Is this intended? Or did I get lost > in a maze of #ifdefs? The "rdp" is different in the two cases. In the first case, it is one of rcu_sched's per-CPU rcu_data structures, in the second case, it is one of rcu_preempt's per-CPU rcu_data structures. I considered making the first three lines common code, but the extra function bloated more than the duplicate three lines. Perhaps I should have tried harder. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
