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    SubjectRe: Possible nohz-full/RCU issue in arm64 KVM
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    Hi Paul,

    On Fri, 2021-12-17 at 08:07 -0800, Paul E. McKenney wrote:
    > On Fri, Dec 17, 2021 at 04:54:22PM +0100, Paolo Bonzini wrote:
    > > On 12/17/21 15:38, Mark Rutland wrote:
    > > > For example kvm_guest_enter_irqoff() calls guest_enter_irq_off() which calls
    > > > vtime_account_guest_enter(), but kvm_guest_exit_irqoff() doesn't call
    > > > guest_exit_irq_off() and the call to vtime_account_guest_exit() is open-coded
    > > > elsewhere. Also, guest_enter_irq_off() conditionally calls
    > > > rcu_virt_note_context_switch(), but I can't immediately spot anything on the
    > > > exit side that corresponded with that, which looks suspicious.
    > >
    > > rcu_note_context_switch() is a point-in-time notification; it's not strictly
    > > necessary, but it may improve performance a bit by avoiding unnecessary IPIs
    > > from the RCU subsystem.
    > >
    > > There's no benefit from doing it when you're back from the guest, because at
    > > that point the CPU is just running normal kernel code.
    >
    > Do scheduling-clock interrupts from guest mode have the "user" parameter
    > set? If so, that would keep RCU happy.

    Are you referring to the user_mode() check in irqentry_enter()? If so I don't
    think it'll help, arm64 doesn't use that function. It directly calls
    enter_from_{user,kernel}_mode() through its custom entry/exit routines.

    Regards,

    --
    Nicolás Sáenz

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