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Subject: Re: [PATCH 1/7] xen/events: reset affinity of 2-level event initially
To:     =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-2-jgross@suse.com>
 <f89567cf-f954-0d97-087e-5e31bfa6d49d@xen.org>
 <d2017caa-0ea8-ae9d-d9f6-45be3da20688@suse.com>
From:   Julien Grall <julien@xen.org>
Message-ID: <d856a721-4f5f-8f8f-bddd-810213daac9c@xen.org>
Date:   Sat, 6 Feb 2021 12:19:22 +0000
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On 06/02/2021 12:09, Jürgen Groß wrote:
> On 06.02.21 12:20, Julien Grall wrote:
>> 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@vger.kernel.org
>>> Reported-by: Julien Grall <julien@xen.org>
>>> Signed-off-by: Juergen Gross <jgross@suse.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?
> 
> This would need another callback.

Right, this seems to be better than walking over all the CPUs every time 
just for cleaning one bit.

Cheers,

-- 
Julien Grall