kvm: replace machine.availableCond with futexes #11568
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Between when machine.Get() observes that a vCPU is not in state vCPUReady, and when it observes that the same vCPU is not in state vCPUGuest, the vCPU can racily transition from vCPUGuest to vCPUReady in bluepillGuestExit(), causing machine.Get() to block in machine.available.Wait() despite a vCPU being ready. To fix this, drop the vCPU handoff mechanism (vCPUWaiter is still needed for bounce()) and replace machine.available with a sequence counter futex. This also improves utilization by allowing machine.Get() to grab the first vCPU that becomes ready, rather than only the specific vCPU it tags for handoff. PiperOrigin-RevId: 738486108
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kvm: replace machine.availableCond with futexes
Between when machine.Get() observes that a vCPU is not in state vCPUReady, and
when it observes that the same vCPU is not in state vCPUGuest, the vCPU can
racily transition from vCPUGuest to vCPUReady in bluepillGuestExit(), causing
machine.Get() to block in machine.available.Wait() despite a vCPU being ready.
To fix this, drop the vCPU handoff mechanism (vCPUWaiter is still needed for
bounce()) and replace machine.available with a sequence counter futex. This
also improves utilization by allowing machine.Get() to grab the first vCPU that
becomes ready, rather than only the specific vCPU it tags for handoff.