linux-kernel - Re: [Xen-devel] [PATCH v2 1/4] xen/events: Clear cpu_evtchn

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Date:	Fri, 1 May 2015 11:46:42 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<david.vrabel@...rix.com>, <konrad.wilk@...cle.com>
CC:	<xen-devel@...ts.xenproject.org>, <annie.li@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH v2 1/4] xen/events: Clear cpu_evtchn_mask
 before resuming

On 29/04/15 22:10, Boris Ostrovsky wrote:
> When a guest is resumed, the hypervisor may change event channel
> assignments. If this happens and the guest uses 2-level events it
> is possible for the interrupt to be claimed by wrong VCPU since
> cpu_evtchn_mask bits may be stale. This can happen even though
> evtchn_2l_bind_to_cpu() attempts to clear old bits: irq_info that
> is passed in is not necessarily the original one (from pre-migration
> times) but instead is freshly allocated during resume and so any
> information about which CPU the channel was bound to is lost.
> 
> Thus we should clear the mask during resume.
> 
> We also need to make sure that bits for xenstore and console channels
> are set when these two subsystems are resumed. While rebind_evtchn_irq()
> (which is invoked for both of them on a resume) calls irq_set_affinity(),
> the latter will in fact postpone setting affinity until handling the
> interrupt. But because cpu_evtchn_mask will have bits for these two
> cleared we won't be able to take the interrupt.
> 
> With that in mind, we need to bind those two channels explicitly in
> rebind_evtchn_irq(). We will keep irq_set_affinity() so that we have a
> pass through generic irq affinity code later, in case something needs
> to be updated there as well.
> 
> (Also replace cpumask_of(0) with cpumask_of(info->cpu) in
> rebind_evtchn_irq(): it should be set to zero in preceding
> xen_irq_info_evtchn_setup().)
[...]
> @@ -1279,8 +1280,16 @@ void rebind_evtchn_irq(int evtchn, int irq)
>  
>  	mutex_unlock(&irq_mapping_update_lock);
>  
> -	/* new event channels are always bound to cpu 0 */
> -	irq_set_affinity(irq, cpumask_of(0));
> +	bind_vcpu.port = evtchn;
> +	bind_vcpu.vcpu = info->cpu;
> +	if (HYPERVISOR_event_channel_op(EVTCHNOP_bind_vcpu, &bind_vcpu) == 0)
> +		bind_evtchn_to_cpu(evtchn, info->cpu);

Isn't the hypercall is unnecessary since this is a new event channel
it's already bound to VCPU 0 and info->cpu == 0?

I think only the bind_evtchn_to_cpu() call is needed here.

If you agree I can remove the hypercall and apply this series.

> +	else
> +		pr_warn("Failed binding port %d to cpu %d\n",
> +			evtchn, info->cpu);
> +
> +	/* This will be deferred until interrupt is processed */
> +	irq_set_affinity(irq, cpumask_of(info->cpu));

David

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