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

[Oleksandr Andrushchenko]

On 3/14/19 17:40, Boris Ostrovsky wrote:
> 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@xxxxxxxx>
> > > > > >
> > > > > > 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
> > > > > >  ÂÂÂ be cleanly done as the backend still holds those (it was not
> > > > > > told to
> > > > > >  ÂÂÂ free the resources)
> > > > > > - it is not possible to resume driver operation as all the
> > > > > > communication
> > > > > >  ÂÂÂ means with the backned were destroyed by the frontend, thus
> > > > > >  ÂÂÂ making the frontend appear to the guest OS as functional, but
> > > > > >  ÂÂÂ 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.

Hm, so it does work for your use-case, but doesn't for mine.

What would be the best way forward?

1. Implement .resume properly as, for example, block front does [1]

2. Remove .resume completely: this does work as long as backend doesn't 
change anything

I am still a bit unsure if we really need to re-initialize rings, 
re-read front's config from

Xenstore etc - what changes on backend side are expected when we resume 
the front driver?

> -boris

Thank you,

Oleksandr


[1] 
https://elixir.bootlin.com/linux/v5.0.2/source/drivers/block/xen-blkfront.c#L2072