Il 28/07/2014 17:03, David Hildenbrand ha scritto: > Well the difference is, that a STOPPED vcpu can be woken up by non-interrupt > like things (SIGP START) AND a special interrupt (SIGP RESTART - which is like > a "SIPI"++ as it performs a psw exchange - "NMI"). So we basically have two > paths that can lead to a state change. All interrupt bits may be in any > combination (SIGP RESTART interrupts can't be masked out, nor can SIGP START > be > denied). > > The other thing may be that on s390, each vcpu (including itself) can put > another vcpu into the STOPPED state - I assume that this is different for x86 > " > INIT_RECEIVED". For this reason we have to watch out for bad race conditions > (e.g. multiple vcpus working on another vcpu)...
You can do that in x86 by sending an INIT inter-processor interrupt. A SIPI is ignored if the CPU is not in INIT_RECEIVED state. Commit 66450a21f99636af4fafac2afd33f1a40631bc3a introduced the current implementation. - an INIT cancels a previous SIPI; - if both INIT and SIPI are sent, on real hardware you need to have a few hundred microseconds between them, but KVM will reliably process INIT before SIPI. See commit 299018f44ac553dce3caf84df1d14c4764faa279 for an example of the races that can happen. Note that x86 has KVM_MP_STATE_SIPI_RECEIVED state but it is obsolete, we go straight from KVM_MP_STATE_INIT_RECEIVED to KVM_MP_STATE_RUNNABLE.
