netdev - Re: [Xen-devel][PATCH] xen/netfront: Remove unneeded .resume callback

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Date:   Thu, 14 Mar 2019 11:40:11 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        netdev@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, jgross@...e.com,
        sstabellini@...nel.org, davem@...emloft.net
Cc:     Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
Subject: Re: [Xen-devel][PATCH] xen/netfront: Remove unneeded .resume callback

On 3/14/19 11:10 AM, Oleksandr Andrushchenko wrote:
> On 3/14/19 5:02 PM, Boris Ostrovsky wrote:
>> On 3/14/19 10:52 AM, Oleksandr Andrushchenko wrote:
>>> On 3/14/19 4:47 PM, Boris Ostrovsky wrote:
>>>> On 3/14/19 9:17 AM, Oleksandr Andrushchenko wrote:
>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>>>>>
>>>>> Currently on driver resume we remove all the network queues and
>>>>> destroy shared Tx/Rx rings leaving the driver in its current state
>>>>> and never signaling the backend of this frontend's state change.
>>>>> This leads to the number of consequences:
>>>>> - when frontend withdraws granted references to the rings etc. it
>>>>> cannot
>>>>>     be cleanly done as the backend still holds those (it was not
>>>>> told to
>>>>>     free the resources)
>>>>> - it is not possible to resume driver operation as all the
>>>>> communication
>>>>>     means with the backned were destroyed by the frontend, thus
>>>>>     making the frontend appear to the guest OS as functional, but
>>>>>     not really.
>>>> What do you mean? Are you saying that after resume you lose
>>>> connectivity?
>>> Exactly, if you take a look at the .resume callback as it is now
>>> what it does it destroys the rings etc. and never notifies the backend
>>> of that, e.g. it stays in, say, connected state with communication
>>> channels destroyed. It never goes into any other Xen bus state, so
>>> there is
>>> no way its state machine can help recovering.
>>
>> My tree is about a month old so perhaps there is some sort of regression
>> but this certainly works for me. After resume netfront gets
>> XenbusStateInitWait from backend which causes xennet_connect().
> Ah, the difference can be of the way we get the guest enter
> the suspend state. I am making my guest to suspend with:
> echo mem > /sys/power/state
> And then I use an interrupt to the guest (this is a test code)
> to wake it up.
> Could you please share your exact use-case when the guest enters suspend
> and what you do to resume it?


xl save / xl restore

> I can see no way backend may want enter XenbusStateInitWait in my
> use-case
> as it simply doesn't know we want him to.


Yours looks like ACPI path, I don't know how well it was tested TBH.


-boris