Re: Possible nohz-full/RCU issue in arm64 KVM

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:

quoted

On 12/17/21 15:38, Mark Rutland wrote:

quoted

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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