linux-kernel - Re: [PATCH 1/7] xen/events: reset affinity of 2-level event initially

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Message-ID: <f89567cf-f954-0d97-087e-5e31bfa6d49d@xen.org>
Date:   Sat, 6 Feb 2021 11:20:16 +0000
From:   Julien Grall <julien@....org>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/7] xen/events: reset affinity of 2-level event initially

Hi Juergen,

On 06/02/2021 10:49, Juergen Gross wrote:
> When creating a new event channel with 2-level events the affinity
> needs to be reset initially in order to avoid using an old affinity
> from earlier usage of the event channel port.
> 
> The same applies to the affinity when onlining a vcpu: all old
> affinity settings for this vcpu must be reset. As percpu events get
> initialized before the percpu event channel hook is called,
> resetting of the affinities happens after offlining a vcpu (this is
> working, as initial percpu memory is zeroed out).
> 
> Cc: stable@...r.kernel.org
> Reported-by: Julien Grall <julien@....org>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>   drivers/xen/events/events_2l.c | 20 ++++++++++++++++++++
>   1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/xen/events/events_2l.c b/drivers/xen/events/events_2l.c
> index da87f3a1e351..23217940144a 100644
> --- a/drivers/xen/events/events_2l.c
> +++ b/drivers/xen/events/events_2l.c
> @@ -47,6 +47,16 @@ static unsigned evtchn_2l_max_channels(void)
>   	return EVTCHN_2L_NR_CHANNELS;
>   }
>   
> +static int evtchn_2l_setup(evtchn_port_t evtchn)
> +{
> +	unsigned int cpu;
> +
> +	for_each_online_cpu(cpu)
> +		clear_bit(evtchn, BM(per_cpu(cpu_evtchn_mask, cpu)));

The bit corresponding to the event channel can only be set on a single 
CPU. Could we avoid the loop and instead clear the bit while closing the 
port?

Cheers,

-- 
Julien Grall