From: KY Srinivasan <kys@microsoft.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>, Olaf Hering <olaf@aepfle.de>
CC: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>
Subject: RE: move hyperv CHANNELMSG_UNLOAD from crashed kernel to kdump kernel
Thread-Topic: move hyperv CHANNELMSG_UNLOAD from crashed kernel to kdump
 kernel
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Date: Fri, 16 Dec 2016 00:51:51 +0000
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> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> Sent: Thursday, December 15, 2016 6:32 AM
> To: Olaf Hering <olaf@aepfle.de>
> Cc: KY Srinivasan <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
> 
> Olaf Hering <olaf@aepfle.de> writes:
> 
> > On Thu, Dec 15, Vitaly Kuznetsov wrote:
> >
> >> vmbus_wait_for_unload() may be receiving a message (not necessarily
> the
> >> CHANNELMSG_UNLOAD_RESPONSE, we may see some other message)
> on the same
> >> CPU it runs and in this case wrmsrl() makes sense. In other cases it
> >> does nothing (neither good nor bad).
> >
> > If that other cpu has interrupts disabled it may not process a pending
> > msg (the response may be stuck in the host queue?), and the loop can not
> > kick the other cpus queue if a wrmsrl is just valid for the current cpu.
> > If thats true, the response will not arrive in the loop.
> >
> 
> In case interrupts get permanently disabled on the CPU which is supposed
> to receive the CHANNELMSG_UNLOAD_RESPONSE message *and* there is
> some
> other message pedning in the slot for that CPU we'll hang. We may try to
> overcome this by sending NMIs but this is getting more and more
> complicated...
> 
> I'd like to see a simple fix from Hyper-V host team: always deliver
> CHANNELMSG_UNLOAD_RESPONSE reply to the cpu which sent
> CHANNELMSG_UNLOAD
> request. This would allow us to remove all the craziness.

Agreed; I will give this feedback to the Hyper-V guys.

K. Y
> 
> --
>   Vitaly