On 10/02/2017 10:50, Paolo Bonzini wrote:
> The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
> a VCPU out of KVM_RUN through a POSIX signal. A signal is attached
> to a dummy signal handler; by blocking the signal outside KVM_RUN and
> unblocking it inside, this possible race is closed:
>
> VCPU thread service thread
> --------------------------------------------------------------
> check flag
> set flag
> raise signal
> (signal handler does nothing)
> KVM_RUN
>
> However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
> tsk->sighand->siglock on every KVM_RUN. This lock is often on a
> remote NUMA node, because it is on the node of a thread's creator.
> Taking this lock can be very expensive if there are many userspace
> exits (as is the case for SMP Windows VMs without Hyper-V reference
> time counter).
>
> As an alternative, we can put the flag directly in kvm_run so that
> KVM can see it:
>
> VCPU thread service thread
> --------------------------------------------------------------
> raise signal
> signal handler
> set run->immediate_exit
> KVM_RUN
> check run->immediate_exit
>
> This is what the last patch in this series does, together with
> the corresponding kernel API. The first six patches are a long
> detour in the signal handling code, moving KVM-specific stuff
> from cpus.c to kvm-all.c so that we have a better hook point for
> KVM_CAP_IMMEDIATE_EXIT (patches 1-3, 6).
>
> Because KVM_SET_SIGNAL_MASK is also unblocking SIGBUS so that
> BUS_MCEERR_AR actions can be delivered via sigwait, we also have
> to rewrite it (patch 4-5) to avoid sigwait, stowing the machine check
> exception as soon as KVM_RUN exits and process it outside the signal
> handler.
>
> The seventh patch would of course be split between a linux-headers
> update and the rest.
>
> Paolo
>
> ps: As an aside, I finally figured out how to test machine check
> forwarding and I hope to write something about it.
This is needed to test these patches with the final version of the KVM part:
diff --git a/kvm-all.c b/kvm-all.c
index 083143f..c79833e 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1922,6 +1922,7 @@ static void kvm_eat_signals(CPUState *cpu)
int r;
if (kvm_immediate_exit) {
+ atomic_set(&cpu->kvm_run->immediate_exit, 0);
return;
}
Paolo
