linux-kernel - Re: [PATCH RFC V2 3/5] kvm hypervisor : Add two hypercalls to support pv-ticketlock

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Date:	Mon, 24 Oct 2011 17:57:34 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	x86@...nel.org, KVM <kvm@...r.kernel.org>,
	Dave Jiang <dave.jiang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Xen <xen-devel@...ts.xensource.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Suzuki Poulose <suzuki@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC V2 3/5] kvm hypervisor : Add two hypercalls to
 support pv-ticketlock

* Avi Kivity <avi@...hat.com> [2011-10-24 12:14:21]:

> > +/*
> > + * kvm_pv_wait_for_kick_op : Block until kicked by either a KVM_HC_KICK_CPU
> > + * hypercall or a event like interrupt.
> > + *
> > + * @vcpu : vcpu which is blocking.
> > + */
> > +static void kvm_pv_wait_for_kick_op(struct kvm_vcpu *vcpu)
> > +{

[snip]

> > +}
> 
> This hypercall can be replaced by a HLT instruction, no?

Good point. Assuming yield_on_hlt=1, that would allow the vcpu to be put
to sleep and let other vcpus make progress.

I guess with that change, we can also dropthe need for other hypercall
introduced in this patch (kvm_pv_kick_cpu_op()). Essentially a vcpu sleeping 
because of HLT instruction can be woken up by a IPI issued by vcpu releasing a
lock.

- vatsa

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