[Qemu-devel] [BUG/RFC] INIT IPI lost when VM starts

[Herongguang (Stephen)]

Hi,
We encountered a problem that when a domain starts, seabios failed to online a 
vCPU.

After investigation, we found that the reason is in kvm-kmod, KVM_APIC_INIT bit 
in
vcpu->arch.apic->pending_events was overwritten by qemu, and thus an INIT IPI 
sent
to AP was lost. Qemu does this since libvirtd sends a ‘query-cpus’ qmp command 
to qemu
on VM start.

In qemu, qmp_query_cpus-> cpu_synchronize_state-> kvm_cpu_synchronize_state->
do_kvm_cpu_synchronize_state, qemu gets registers/vcpu_events from kvm-kmod and
sets cpu->kvm_vcpu_dirty to true, and vcpu thread in qemu will call
kvm_arch_put_registers if cpu->kvm_vcpu_dirty is true, thus pending_events is
overwritten by qemu.

I think there is no need for qemu to set cpu->kvm_vcpu_dirty to true after 
‘query-cpus’,
and  kvm-kmod should not clear KVM_APIC_INIT unconditionally. And I am not sure 
whether
it is OK for qemu to set cpu->kvm_vcpu_dirty in do_kvm_cpu_synchronize_state in 
each caller.

What’s your opinion?