Messages in this thread | | | Subject | Re: [PATCH 1/3] KVM: s390: clear kicked_mask before sleeping again | From | Christian Borntraeger <> | Date | Wed, 20 Oct 2021 08:03:40 +0200 |
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Am 20.10.21 um 07:35 schrieb Claudio Imbrenda: > On Tue, 19 Oct 2021 19:53:59 +0200 > Halil Pasic <pasic@linux.ibm.com> wrote: > >> The idea behind kicked mask is that we should not re-kick a vcpu that >> is already in the "kick" process, i.e. that was kicked and is >> is about to be dispatched if certain conditions are met. >> >> The problem with the current implementation is, that it assumes the >> kicked vcpu is going to enter SIE shortly. But under certain >> circumstances, the vcpu we just kicked will be deemed non-runnable and >> will remain in wait state. This can happen, if the interrupt(s) this >> vcpu got kicked to deal with got already cleared (because the interrupts >> got delivered to another vcpu). In this case kvm_arch_vcpu_runnable() >> would return false, and the vcpu would remain in kvm_vcpu_block(), >> but this time with its kicked_mask bit set. So next time around we >> wouldn't kick the vcpu form __airqs_kick_single_vcpu(), but would assume >> that we just kicked it. >> >> Let us make sure the kicked_mask is cleared before we give up on >> re-dispatching the vcpu. >> >> Signed-off-by: Halil Pasic <pasic@linux.ibm.com> >> Reported-by: Matthew Rosato <mjrosato@linux.ibm.com> >> Fixes: 9f30f6216378 ("KVM: s390: add gib_alert_irq_handler()") >> --- >> arch/s390/kvm/kvm-s390.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c >> index 6a6dd5e1daf6..1c97493d21e1 100644 >> --- a/arch/s390/kvm/kvm-s390.c >> +++ b/arch/s390/kvm/kvm-s390.c >> @@ -3363,6 +3363,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu) >> >> int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu) >> { >> + clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask); > > so, you unconditionally clear the flag, before knowing if the vCPU is > runnable? > > from your description I would have expected to only clear the bit if > the vCPU is not runnable. > > would things break if we were to try to kick the vCPU again after > clearing the bit, but before dispatching it?
The whole logic is just an optimization to avoid unnecessary wakeups. When the bit is set a wakup might be omitted. I prefer to do an unneeded wakeup over not doing a wakeup so I think over-clearing is safer. In fact, getting rid of this micro-optimization would be a valid alternative. > >> return kvm_s390_vcpu_has_irq(vcpu, 0); >> } >> >
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