Re: [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Dec 07, 2011 at 05:24:59PM +0530, Raghavendra K T wrote:
> On 12/07/2011 04:18 PM, Marcelo Tosatti wrote:
> >On Wed, Nov 30, 2011 at 02:29:59PM +0530, Raghavendra K T wrote:
> >>
> >>+/*
> >>+ * kvm_pv_kick_cpu_op:  Kick a vcpu.
> >>+ *
> >>+ * @cpu - vcpu to be kicked.
> >>+ */
> >>+static void kvm_pv_kick_cpu_op(struct kvm *kvm, int cpu)
> >>+{
> >>+	struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, cpu);
> >>+	struct kvm_mp_state mp_state;
> >>+
> >>+	mp_state.mp_state = KVM_MP_STATE_RUNNABLE;
> >
> >Since vcpu->mp_state is not protected by a lock, this is potentially racy. For example:
> >
> >CPU0						CPU1
> >kvm_pv_kick_cpu_op				running vcpuN
> >vcpuN->mp_state = KVM_MP_STATE_RUNNABLE;
> >						kvm_emulate_halt
> >						vcpuN->mp_state = KVM_MP_STATE_HALTED
> >
> >Is it harmless to lose a kick?
> >
> 
> Yes you are right. It was potentially racy and it was harmful too!.
> I had observed that it was stalling the CPU before I introduced
> kicked flag.
> 
> But now,
> 
> vcpu->kicked = 1  ==> kvm_make_request(KVM_REQ_UNHALT, vcpu); ==>

Ok, please use a more descriptive name, such as "pvlock_kicked" or
something.

> 
> __vcpu_run() ==> kvm_check_request(KVM_REQ_UNHALT, vcpu) ==>
> 
> vcpuN->mp_state = KVM_MP_STATE_RUNNABLE; so eventually we will end up
> in RUNNABLE.
> 
> Also Avi pointed that, logically kvm_arch_vcpu_ioctl_set_mpstate should
> be called only in vcpu thread, so after further debugging, I noticed
> that,  setting vcpuN->mp_state = KVM_MP_STATE_RUNNABLE; is not
> necessary.
> I 'll remove that in the next patch. Thanks for pointing.

In fact you don't need kvm_arch_vcpu_ioctl_set_mpstate either, only the
new "kicked" flag.

> 
> 
>