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). -- Alex Bennée -- 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
