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    SubjectRe: RCU+KVM: making CPU guest mode a quiescent state.
    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.
    Definitely. This will allow other CPUs to complete rcu barrier much
    earlier.

    >
    > 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?
    >
    One of them works on rcu_sched_data another on rcu_preempt_data as far
    as I see.

    --
    Gleb.


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