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.
Paolo
