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

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Date:	Wed, 7 Dec 2011 10:33:30 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Raghavendra K T <raghukt@...ux.vnet.ibm.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>, KVM <kvm@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Gleb Natapov <gleb@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Xen <xen-devel@...ts.xensource.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Suzuki Poulose <suzuki@...ux.vnet.ibm.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM
 hypervisor to support pv-ticketlocks

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.

> 
> 
> 			
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