netdev - Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks for PM suspend and hibernation

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Message-ID: <20200221092219.GU4679@Air-de-Roger>
Date:   Fri, 21 Feb 2020 10:22:19 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     "Durrant, Paul" <pdurrant@...zon.co.uk>
CC:     "Agarwal, Anchal" <anchalag@...zon.com>,
        "Valentin, Eduardo" <eduval@...zon.com>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "x86@...nel.org" <x86@...nel.org>,
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        "Kamata, Munehisa" <kamatam@...zon.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
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        "jgross@...e.com" <jgross@...e.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
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        "davem@...emloft.net" <davem@...emloft.net>,
        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks for
 PM suspend and hibernation

On Thu, Feb 20, 2020 at 05:01:52PM +0000, Durrant, Paul wrote:
> > > Hopefully what I said above illustrates why it may not be 100% common.
> > 
> > Yes, that's fine. I don't expect it to be 100% common (as I guess
> > that the hooks will have different prototypes), but I expect
> > that routines can be shared, and that the approach taken can be the
> > same.
> > 
> > For example one necessary difference will be that xenbus initiated
> > suspend won't close the PV connection, in case suspension fails. On PM
> > suspend you seem to always close the connection beforehand, so you
> > will always have to re-negotiate on resume even if suspension failed.
> > 
> > What I'm mostly worried about is the different approach to ring
> > draining. Ie: either xenbus is changed to freeze the queues and drain
> > the shared rings, or PM uses the already existing logic of not
> > flushing the rings an re-issuing in-flight requests on resume.
> > 
> 
> Yes, that's needs consideration. I don’t think the same semantic can be suitable for both. E.g. in a xen-suspend we need to freeze with as little processing as possible to avoid dirtying RAM late in the migration cycle, and we know that in-flight data can wait. But in a transition to S4 we need to make sure that at least all the in-flight blkif requests get completed, since they probably contain bits of the guest's memory image and that's not going to get saved any other way.

Thanks, that makes sense and something along this lines should be
added to the commit message IMO.

Wondering about S4, shouldn't we expect the queues to already be
empty? As any subsystem that wanted to store something to disk should
make sure requests have been successfully completed before
suspending.

Thanks, Roger.