From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Olaf Hering <olaf@aepfle.de>
Cc: kys@microsoft.com, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, devel@linuxdriverproject.org
Subject: Re: move hyperv CHANNELMSG_UNLOAD from crashed kernel to kdump kernel
References: <20161207085110.GC1618@aepfle.de>
Date: Thu, 15 Dec 2016 11:26:48 +0100
In-Reply-To: <20161207085110.GC1618@aepfle.de> (Olaf Hering's message of "Wed,
        7 Dec 2016 09:51:10 +0100")
Message-ID: <87r3594hef.fsf@vitty.brq.redhat.com>
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Olaf Hering <olaf@aepfle.de> writes:

> KY,
>
> if a hyperv VM crashes alot of work must be done to prepare the
> environment for the kdump kernel. This approach is different compared to
> all the other VM types, or baremetal. Since the just crashed kernel is
> per definition unreliable all that work should be done within the kdump
> kernel because I think a reliable environment exists only there.
>
> Was it ever considered to do the CHANNELMSG_UNLOAD /
> CHANNELMSG_UNLOAD_RESPONSE work during boot, instead of doing it before
> starting the kexec/kdump kernel?

Sorry guys I missed the discussion, I was on vacation.

I see a number of minor but at least one major issue against such move:
At least for some Hyper-V versions (2012R2 for example)
CHANNELMSG_UNLOAD_RESPONSE is delivered to the CPU which initially sent 
CHANNELMSG_REQUESTOFFERS and on kdump we may not have this CPU up as
we usually do kdump with nr_cpus=1 (and on the CPU which crashed). 

Minor issue is the necessity preserve the information about
message/events pages across kexec.

>
> What would it take to prepare the runtime environment during boot?
> Does the newly booted kernel need any info from the previous kernel,
> something that cant be determined during boot? If yes, how can such info
> be passed from the old kernel to the new kernel?
>
> Olaf
>

-- 
  Vitaly