On Fri, Aug 01, 2014 at 10:48:36AM +0100, Alex Bennée wrote: > > Christoffer Dall writes: > > > On Thu, Jul 31, 2014 at 05:45:28PM +0100, Peter Maydell wrote: > >> On 31 July 2014 17:38, Christoffer Dall <christoffer.dall@xxxxxxxxxx> wrote: > >> >> > If we are not complaining when setting the pause value to false if it > >> >> > was true before, then we probably also need to wake up the thread in > >> >> > case this is called from another thread, right? > >> >> > > >> >> > or perhaps we should just return an error if you're trying to un-pause a > >> >> > CPU through this interface, hmmmm. > >> >> > >> >> Wouldn't it be an error to mess with any register when the system is not > >> >> in a quiescent state? I was assuming that the wake state is dealt with > >> >> when the run loop finally restarts. > >> >> > >> > > >> > The ABI doesn't really define it as an error (the ABI doesn't enforce > >> > anything right now) so the question is, does it ever make sense to clear > >> > the pause flag through this ioctl? If not, I think we should just err > >> > on the side of caution and specify in the docs that this is not > >> > supported and return an error. > >> > >> Consider the case where the reset state of the system is > >> "CPU 0 running, CPUs 1..N stopped", and we're doing an > >> incoming migration to a state where all CPUs are running. > >> In that case we'll be using this ioctl to clear the pause flag, > >> right? (We'll also obviously need to set the PC and other > >> register state correctly before resuming the guest.) > >> > > Doh, you're right, I somehow had it in my mind that when you send the > > thread a signal, the pause flag would be cleared, but that goes against > > the whole idea of a CPU being turned off for KVM. > > > > But wouldn't we then have to also wake up the thread when clearing the > > pause flag? It feels strange that the ioctl can clear the pause flag, > > but keep the thread on a wake-queue, and then userspace has to send the > > thread a signal of some sort to wake it up? > <snip> > > Isn't the vCPU off the wait-queue by definition if the ioctl exits and > you go through the KVM_SET_ONE_REG stuff? > > Once you re-enter the KVM_RUN ioctl it sees the pause_flag as cleared > and falls straight through into kvm_guest_enter() otherwise it will > again wait on wait_event_interruptible(*wq, !vcpu->arch.pause). > Yeah, you're right, I forgot we grab the vcpu->mutex. -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
