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Re: [Xen-devel] [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks
- To: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
- From: Raghavendra K T <raghukt@xxxxxxxxxxxxxxxxxx>
- Date: Wed, 07 Dec 2011 17:24:59 +0530
- Cc: x86@xxxxxxxxxx, Gleb Natapov <gleb@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Xen <xen-devel@xxxxxxxxxxxxxxxxxxx>, Dave Jiang <dave.jiang@xxxxxxxxx>, KVM <kvm@xxxxxxxxxxxxxxx>, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Sasha Levin <levinsasha928@xxxxxxxxx>, Sedat Dilek <sedat.dilek@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Yinghai Lu <yinghai@xxxxxxxxxx>, Greg Kroah-Hartman <gregkh@xxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>, Suzuki Poulose <suzuki@xxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 12 Dec 2011 14:39:50 +0000
- List-id: Xen developer discussion <xen-devel.lists.xensource.com>
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); ==>
__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.
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