Date:   Thu, 20 Feb 2020 17:01:52 +0000
From:   "Durrant, Paul" <pdurrant@...zon.co.uk>
To:     Roger Pau Monné <roger.pau@...rix.com>
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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        "pavel@....cz" <pavel@....cz>, "hpa@...or.com" <hpa@...or.com>,
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        "sstabellini@...nel.org" <sstabellini@...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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        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
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        "vkuznets@...hat.com" <vkuznets@...hat.com>,
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        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: RE: [Xen-devel] [RFC PATCH v3 06/12] xen-blkfront: add callbacks for
 PM suspend and hibernation

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@...rix.com>
> Sent: 20 February 2020 16:49
> To: Durrant, Paul <pdurrant@...zon.co.uk>
> Cc: Agarwal, Anchal <anchalag@...zon.com>; Valentin, Eduardo
> <eduval@...zon.com>; len.brown@...el.com; peterz@...radead.org;
> benh@...nel.crashing.org; x86@...nel.org; linux-mm@...ck.org;
> pavel@....cz; hpa@...or.com; tglx@...utronix.de; sstabellini@...nel.org;
> fllinden@...ozn.com; Kamata, Munehisa <kamatam@...zon.com>;
> mingo@...hat.com; xen-devel@...ts.xenproject.org; Singh, Balbir
> <sblbir@...zon.com>; axboe@...nel.dk; konrad.wilk@...cle.com;
> bp@...en8.de; boris.ostrovsky@...cle.com; jgross@...e.com;
> netdev@...r.kernel.org; linux-pm@...r.kernel.org; rjw@...ysocki.net;
> linux-kernel@...r.kernel.org; vkuznets@...hat.com; 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 04:23:13PM +0000, Durrant, Paul wrote:
> > > -----Original Message-----
> > > From: Roger Pau Monné <roger.pau@...rix.com>
> > > Sent: 20 February 2020 15:45
> > > To: Durrant, Paul <pdurrant@...zon.co.uk>
> > > Cc: Agarwal, Anchal <anchalag@...zon.com>; Valentin, Eduardo
> > > <eduval@...zon.com>; len.brown@...el.com; peterz@...radead.org;
> > > benh@...nel.crashing.org; x86@...nel.org; linux-mm@...ck.org;
> > > pavel@....cz; hpa@...or.com; tglx@...utronix.de;
> sstabellini@...nel.org;
> > > fllinden@...ozn.com; Kamata, Munehisa <kamatam@...zon.com>;
> > > mingo@...hat.com; xen-devel@...ts.xenproject.org; Singh, Balbir
> > > <sblbir@...zon.com>; axboe@...nel.dk; konrad.wilk@...cle.com;
> > > bp@...en8.de; boris.ostrovsky@...cle.com; jgross@...e.com;
> > > netdev@...r.kernel.org; linux-pm@...r.kernel.org; rjw@...ysocki.net;
> > > linux-kernel@...r.kernel.org; vkuznets@...hat.com;
> 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 08:54:36AM +0000, Durrant, Paul wrote:
> > > > > -----Original Message-----
> > > > > From: Xen-devel <xen-devel-bounces@...ts.xenproject.org> On Behalf
> Of
> > > > > Roger Pau Monné
> > > > > Sent: 20 February 2020 08:39
> > > > > To: Agarwal, Anchal <anchalag@...zon.com>
> > > > > Cc: Valentin, Eduardo <eduval@...zon.com>; len.brown@...el.com;
> > > > > peterz@...radead.org; benh@...nel.crashing.org; x86@...nel.org;
> linux-
> > > > > mm@...ck.org; pavel@....cz; hpa@...or.com; tglx@...utronix.de;
> > > > > sstabellini@...nel.org; fllinden@...ozn.com; Kamata, Munehisa
> > > > > <kamatam@...zon.com>; mingo@...hat.com; xen-
> > > devel@...ts.xenproject.org;
> > > > > Singh, Balbir <sblbir@...zon.com>; axboe@...nel.dk;
> > > > > konrad.wilk@...cle.com; bp@...en8.de; boris.ostrovsky@...cle.com;
> > > > > jgross@...e.com; netdev@...r.kernel.org; linux-pm@...r.kernel.org;
> > > > > rjw@...ysocki.net; linux-kernel@...r.kernel.org;
> vkuznets@...hat.com;
> > > > > 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
> > > > >
> > > > > Thanks for this work, please see below.
> > > > >
> > > > > On Wed, Feb 19, 2020 at 06:04:24PM +0000, Anchal Agarwal wrote:
> > > > > > On Tue, Feb 18, 2020 at 10:16:11AM +0100, Roger Pau Monné wrote:
> > > > > > > On Mon, Feb 17, 2020 at 11:05:53PM +0000, Anchal Agarwal
> wrote:
> > > > > > > > On Mon, Feb 17, 2020 at 11:05:09AM +0100, Roger Pau Monné
> wrote:
> > > > > > > > > On Fri, Feb 14, 2020 at 11:25:34PM +0000, Anchal Agarwal
> > > wrote:
> > > > > > > > Quiescing the queue seemed a better option here as we want
> to
> > > make
> > > > > sure ongoing
> > > > > > > > requests dispatches are totally drained.
> > > > > > > > I should accept that some of these notion is borrowed from
> how
> > > nvme
> > > > > freeze/unfreeze
> > > > > > > > is done although its not apple to apple comparison.
> > > > > > >
> > > > > > > That's fine, but I would still like to requests that you use
> the
> > > same
> > > > > > > logic (as much as possible) for both the Xen and the PM
> initiated
> > > > > > > suspension.
> > > > > > >
> > > > > > > So you either apply this freeze/unfreeze to the Xen suspension
> > > (and
> > > > > > > drop the re-issuing of requests on resume) or adapt the same
> > > approach
> > > > > > > as the Xen initiated suspension. Keeping two completely
> different
> > > > > > > approaches to suspension / resume on blkfront is not suitable
> long
> > > > > > > term.
> > > > > > >
> > > > > > I agree with you on overhaul of xen suspend/resume wrt blkfront
> is a
> > > > > good
> > > > > > idea however, IMO that is a work for future and this patch
> series
> > > should
> > > > > > not be blocked for it. What do you think?
> > > > >
> > > > > It's not so much that I think an overhaul of suspend/resume in
> > > > > blkfront is needed, it's just that I don't want to have two
> completely
> > > > > different suspend/resume paths inside blkfront.
> > > > >
> > > > > So from my PoV I think the right solution is to either use the
> same
> > > > > code (as much as possible) as it's currently used by Xen initiated
> > > > > suspend/resume, or to also switch Xen initiated suspension to use
> the
> > > > > newly introduced code.
> > > > >
> > > > > Having two different approaches to suspend/resume in the same
> driver
> > > > > is a recipe for disaster IMO: it adds complexity by forcing
> developers
> > > > > to take into account two different suspend/resume approaches when
> > > > > there's no need for it.
> > > >
> > > > I disagree. S3 or S4 suspend/resume (or perhaps we should call them
> > > power state transitions to avoid confusion) are quite different from
> Xen
> > > suspend/resume.
> > > > Power state transitions ought to be, and indeed are, visible to the
> > > software running inside the guest. Applications, as well as drivers,
> can
> > > receive notification and take whatever action they deem appropriate.
> > > > Xen suspend/resume OTOH is used when a guest is migrated and the
> code
> > > should go to all lengths possible to make any software running inside
> the
> > > guest (other than Xen specific enlightened code, such as PV drivers)
> > > completely unaware that anything has actually happened.
> > >
> > > So from what you say above PM state transitions are notified to all
> > > drivers, and Xen suspend/resume is only notified to PV drivers, and
> > > here we are speaking about blkfront which is a PV driver, and should
> > > get notified in both cases. So I'm unsure why the same (or at least
> > > very similar) approach can't be used in both cases.
> > >
> > > The suspend/resume approach proposed by this patch is completely
> > > different than the one used by a xenbus initiated suspend/resume, and
> > > I don't see a technical reason that warrants this difference.
> > >
> >
> > Within an individual PV driver it may well be ok to use common
> mechanisms for connecting to the backend but issues will arise if any
> subsequent action is visible to the guest. E.g. a network frontend needs
> to issue gratuitous ARPs without anything else in the network stack (or
> monitoring the network stack) knowing that it has happened.
> >
> > > I'm not saying that the approach used here is wrong, it's just that I
> > > don't see the point in having two different ways to do suspend/resume
> > > in the same driver, unless there's a technical reason for it, which I
> > > don't think has been provided.
> >
> > The technical justification is that the driver needs to know what kind
> of suspend or resume it is doing, so that it doesn't do the wrong thing.
> There may also be differences in the state of the system e.g. in Windows,
> at least some of the resume-from-xen-suspend code runs with interrupts
> disabled (which is necessary to make sure enough state is restored before
> things become visible to other kernel code).
> >
> > >
> > > I would be fine with switching xenbus initiated suspend/resume to also
> > > use the approach proposed here: freeze the queues and drain the shared
> > > rings before suspending.
> > >
> >
> > I think abstracting away at the xenbus level to some degree is probably
> feasible, but some sort of flag should be passed to the individual drivers
> so they know what circumstances they are operating under.
> >
> > > > So, whilst it may be possible to use common routines to, for
> example,
> > > re-establish PV frontend/backend communication, PV frontend code
> should be
> > > acutely aware of the circumstances they are operating in. I can cite
> > > example code in the Windows PV driver, which have supported guest
> S3/S4
> > > power state transitions since day 1.
> > >
> > > Hm, please bear with me, as I'm not sure I fully understand. Why isn't
> > > the current suspend/resume logic suitable for PM transitions?
> > >
> >
> > I don’t know the details for Linux but it may well be to do with
> assumptions made about the system e.g. the ability to block waiting for
> something to happen on another CPU (which may have already been quiesced
> in a PM context).
> >
> > > As said above, I'm happy to switch xenbus initiated suspend/resume to
> > > use the logic in this patch, but unless there's a technical reason for
> > > it I don't see why blkfront should have two completely different
> > > approaches to suspend/resume depending on whether it's a PM or a
> > > xenbus state change.
> > >
> >
> > 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.

  Paul

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