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>
        <87r3594hef.fsf@vitty.brq.redhat.com>
        <20161215103402.GA6336@aepfle.de>
        <87mvfx4g4y.fsf@vitty.brq.redhat.com>
        <20161215125139.GC6336@aepfle.de>
Date: Thu, 15 Dec 2016 14:28:04 +0100
In-Reply-To: <20161215125139.GC6336@aepfle.de> (Olaf Hering's message of "Thu,
        15 Dec 2016 13:51:39 +0100")
Message-ID: <87bmwd490b.fsf@vitty.brq.redhat.com>
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Olaf Hering <olaf@aepfle.de> writes:

> On Thu, Dec 15, Vitaly Kuznetsov wrote:
>
>> We actually need to read the reply and empty the message slot to make
>> unload happen. And reading on a different CPU may not work, see:
>> 
>> http://driverdev.linuxdriverproject.org/pipermail/driverdev-devel/2016-December/097330.html
>
> Does the following sentences mean the vmbus_signal_eom in
> vmbus_wait_for_unload is a noop because the wrmsrl() is expected to
> happen on the other cpu instead of the current cpu?
>
> ...
> - When we read the message we need to clear the slot and signal the fact
>   to the hypervisor. In case there are more messages to this CPU pending
>   the hypervisor will deliver the next message. The signaling is done by
>   writing to an MSR so this can only be done on the appropriate CPU.
> ...
>

-> K. Y., but these words were written before I implemented
vmbus_wait_for_unload(), to me they just explain how we read messages.

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).

-- 
  Vitaly