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 12:14:21 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	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>,
	Srivatsa Vaddagiri <vatsa@...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

On 10/23/2011 09:05 PM, Raghavendra K T wrote:
> Add two hypercalls to KVM hypervisor to support pv-ticketlocks.
>     
> KVM_HC_WAIT_FOR_KICK blocks the calling vcpu until another vcpu kicks it or it
> is woken up because of an event like interrupt.
>    
> KVM_HC_KICK_CPU allows the calling vcpu to kick another vcpu.
>     
> The presence of these hypercalls is indicated to guest via
> KVM_FEATURE_WAIT_FOR_KICK/KVM_CAP_WAIT_FOR_KICK.
>
> Qemu needs a corresponding patch to pass up the presence of this feature to 
> guest via cpuid. Patch to qemu will be sent separately.
>
> There is no Xen/KVM hypercall interface to await kick from.
>     
>  
> +/*
> + * 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)
> +{
> +	DEFINE_WAIT(wait);
> +
> +	/*
> +	 * Blocking on vcpu->wq allows us to wake up sooner if required to
> +	 * service pending events (like interrupts).
> +	 *
> +	 * Also set state to TASK_INTERRUPTIBLE before checking vcpu->kicked to
> +	 * avoid racing with kvm_pv_kick_cpu_op().
> +	 */
> +	prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> +
> +	/*
> +	 * Somebody has already tried kicking us. Acknowledge that
> +	 * and terminate the wait.
> +	 */
> +	if (vcpu->kicked) {
> +		vcpu->kicked = 0;
> +		goto end_wait;
> +	}
> +
> +	/* Let's wait for either KVM_HC_KICK_CPU or someother event
> +	 * to wake us up.
> +	 */
> +
> +	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> +	schedule();
> +	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> +
> +end_wait:
> +	finish_wait(&vcpu->wq, &wait);
> +}

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

I'm pretty sure this misses a lot of stuff from kvm_vcpu_block().

> +
> +/*
> + * 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);
> +

Is the vcpu number meaningful?  We should reuse an existing identifier
like the APIC ID.

> +	if (vcpu) {
> +		vcpu->kicked = 1;

Need to use smp memory barriers here.

> +		wake_up_interruptible(&vcpu->wq);
> +	}
> +}
> +
>  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long nr, a0, a1, a2, a3, ret;
>

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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