On Fri, Dec 17, 2021 at 06:23:32PM +0100, Paolo Bonzini wrote: > On 12/17/21 18:12, Paul E. McKenney wrote: > > On Fri, Dec 17, 2021 at 06:02:23PM +0100, Paolo Bonzini wrote: > > > On 12/17/21 17:45, Paul E. McKenney wrote: > > > > On Fri, Dec 17, 2021 at 05:34:04PM +0100, Paolo Bonzini wrote: > > > > > On 12/17/21 17:07, Paul E. McKenney wrote: > > > > > > > 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. > > > > > > > > > > No, thread is in supervisor mode. But after every interrupt (timer tick or > > > > > anything), one of three things can happen: > > > > > > > > > > * KVM will go around the execution loop and invoke rcu_note_context_switch() > > > > > again > > > > > > > > > > * or KVM will go back to user space > > > > > > > > Here "user space" is a user process as opposed to a guest OS? > > > > > > Yes, that code runs from ioctl(KVM_RUN) and the ioctl will return to the > > > calling process. > > > > Intriguing. A user process within the guest OS or a user process outside > > of any guest OS, that is, within the host? > > A user process on the host. The guest vCPU is nothing special: it's just a > user thread that occasionally lets the guest run by invoking the KVM_RUN > ioctl. Hopefully, KVM_RUN will be where that user thread will spend most of > the time so the guest runs at full steam. KVM_RUN is the place where you > have the code that Nicolas and Mark were discussing. > > From the point of view of the kernel however the thread is always in kernel > mode when it runs the guest, because any interrupt will be recognized while > still in the ioctl. > > (I'll add that from the point of view of the scheduler, there's no > difference between a CPU-bound guest and a "normal" CPU-bound process on the > host, e.g. wasting time with "for(;;)" or calculating digits of PI is the > same no matter if you're doing it in the guest or in the host. Likewise for > I/O-bound guests; e.g. doing "hlt" or "wfi" constantly in the guest looks > exactly the same to the scheduler as a process that spends its time in the > poll() system call). Thank you for the explanation! Thanx, Paul _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
