linux-kernel - Re: move hyperv CHANNELMSG_UNLOAD from crashed kernel to kdump kernel

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Message-ID: <20161215103402.GA6336@aepfle.de>
Date:   Thu, 15 Dec 2016 11:34:03 +0100
From:   Olaf Hering <olaf@...fle.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kys@...rosoft.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org
Subject: Re: move hyperv CHANNELMSG_UNLOAD from crashed kernel to kdump kernel

On Thu, Dec 15, Vitaly Kuznetsov wrote:

> 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). 

Since the kdump or kexec kernel will send the unload during boot I would
expect the response to arrive where it was sent, independent from the
number of cpus.

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

I guess this info is stored somewhere, and the relevant gfns can be
preserved across kernels, if we try really hard.

But after looking further at the involved code paths it seems that the
implemnted polling might be good enough to snatch the response. Was the
mdelay(10) just an arbitrary decision? I interpret the comments in
vmbus_signal_eom such that the host may overwrite the response. Perhaps
such thing may happen during the mdelay?

Olaf

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