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Subject: Re: [Xen-devel] [PATCH] x86/xen: Return from panic notifier
To:     Jan Beulich <jbeulich@suse.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        james@dingwall.me.uk, Juergen Gross <jgross@suse.com>
References: <20191001151633.1659-1-boris.ostrovsky@oracle.com>
 <9b3f955c-ad76-601f-2b58-fa9dc4608c72@suse.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <924f41b2-7779-9c56-9b71-56523756ecdc@oracle.com>
Date:   Wed, 2 Oct 2019 09:24:46 -0400
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On 10/2/19 3:40 AM, Jan Beulich wrote:
> On 01.10.2019 17:16, Boris Ostrovsky wrote:
>> Currently execution of panic() continues until Xen's panic notifier
>> (xen_panic_event()) is called at which point we make a hypercall that
>> never returns.
>>
>> This means that any notifier that is supposed to be called later as
>> well as significant part of panic() code (such as pstore writes from
>> kmsg_dump()) is never executed.
> Back at the time when this was introduced into the XenoLinux tree,
> I think this behavior was intentional for at least DomU-s. I wonder
> whether you wouldn't want your change to further distinguish Dom0
> and DomU behavior.

Do you remember what the reason for that was?

I think having ability to call kmsg_dump() on a panic is a useful thing
to have for domUs as well. Besides, there may be other functionality
added post-notifiers in panic() in the future. Or another notifier may
be registered later with the same lowest priority.

Is there a downside in allowing domUs to fall through panic() all the
way to emergency_restart()?

>
>> There is no reason for xen_panic_event() to be this last point in
>> execution since panic()'s emergency_restart() will call into
>> xen_emergency_restart() from where we can perform our hypercall.
> Did you consider, as an alternative, to lower the notifier's
> priority?

I didn't but that wouldn't help with the original problem that James
reported --- we'd still not get to kmsg_dump() call.


-boris