On Wed, Jan 22, 2020 at 11:06:18AM +0100, Greg Kurz wrote: > On Wed, 22 Jan 2020 17:50:28 +1100 > David Gibson <[email protected]> wrote: > > > On Tue, Jan 21, 2020 at 10:32:55AM +0100, Greg Kurz wrote: > > > On Tue, 21 Jan 2020 14:43:32 +1100 > > > David Gibson <[email protected]> wrote: > > > > > > > On Mon, Jan 20, 2020 at 09:04:38AM +0100, Greg Kurz wrote: > > > > > On Fri, 17 Jan 2020 16:44:27 +0100 > > > > > Greg Kurz <[email protected]> wrote: > > > > > > > > > > > On Fri, 17 Jan 2020 19:16:08 +1000 > > > > > > David Gibson <[email protected]> wrote: > > > > > > > > > > > > > On Thu, Jan 16, 2020 at 07:29:02PM +0100, Greg Kurz wrote: > > > > > > > > On Thu, 16 Jan 2020 13:14:35 +0100 > > > > > > > > Greg Kurz <[email protected]> wrote: > > > > > > > > > > > > > > > > > On Thu, 16 Jan 2020 11:37:24 +0100 > > > > > > > > > Laurent Vivier <[email protected]> wrote: > > > > > > > > > > > > > > > > > > > On 16/01/2020 09:48, Greg Kurz wrote: > > > > > > > > > > > On Wed, 15 Jan 2020 19:10:37 +0100 > > > > > > > > > > > Laurent Vivier <[email protected]> wrote: > > > > > > > > > > > > > > > > > > > > > >> Hi, > > > > > > > > > > >> > > > > > > > > > > >> On 15/01/2020 18:48, Greg Kurz wrote: > > > > > > > > > > >>> Migration can potentially race with CAS reboot. If the > > > > > > > > > > >>> migration thread > > > > > > > > > > >>> completes migration after CAS has set spapr->cas_reboot > > > > > > > > > > >>> but before the > > > > > > > > > > >>> mainloop could pick up the reset request and reset the > > > > > > > > > > >>> machine, the > > > > > > > > > > >>> guest is migrated unrebooted and the destination > > > > > > > > > > >>> doesn't reboot it > > > > > > > > > > >>> either because it isn't aware a CAS reboot was needed > > > > > > > > > > >>> (eg, because a > > > > > > > > > > >>> device was added before CAS). This likely result in a > > > > > > > > > > >>> broken or hung > > > > > > > > > > >>> guest. > > > > > > > > > > >>> > > > > > > > > > > >>> Even if it is small, the window between CAS and CAS > > > > > > > > > > >>> reboot is enough to > > > > > > > > > > >>> re-qualify spapr->cas_reboot as state that we should > > > > > > > > > > >>> migrate. Add a new > > > > > > > > > > >>> subsection for that and always send it when a CAS > > > > > > > > > > >>> reboot is pending. > > > > > > > > > > >>> This may cause migration to older QEMUs to fail but it > > > > > > > > > > >>> is still better > > > > > > > > > > >>> than end up with a broken guest. > > > > > > > > > > >>> > > > > > > > > > > >>> The destination cannot honour the CAS reboot request > > > > > > > > > > >>> from a post load > > > > > > > > > > >>> handler because this must be done after the guest is > > > > > > > > > > >>> fully restored. > > > > > > > > > > >>> It is thus done from a VM change state handler. > > > > > > > > > > >>> > > > > > > > > > > >>> Reported-by: Lukáš Doktor <[email protected]> > > > > > > > > > > >>> Signed-off-by: Greg Kurz <[email protected]> > > > > > > > > > > >>> --- > > > > > > > > > > >>> > > > > > > > > > > >> > > > > > > > > > > >> I'm wondering if the problem can be related with the > > > > > > > > > > >> fact that > > > > > > > > > > >> main_loop_should_exit() could release qemu_global_mutex > > > > > > > > > > >> in > > > > > > > > > > >> pause_all_vcpus() in the reset case? > > > > > > > > > > >> > > > > > > > > > > >> 1602 static bool main_loop_should_exit(void) > > > > > > > > > > >> 1603 { > > > > > > > > > > >> ... > > > > > > > > > > >> 1633 request = qemu_reset_requested(); > > > > > > > > > > >> 1634 if (request) { > > > > > > > > > > >> 1635 pause_all_vcpus(); > > > > > > > > > > >> 1636 qemu_system_reset(request); > > > > > > > > > > >> 1637 resume_all_vcpus(); > > > > > > > > > > >> 1638 if (!runstate_check(RUN_STATE_RUNNING) && > > > > > > > > > > >> 1639 > > > > > > > > > > >> !runstate_check(RUN_STATE_INMIGRATE)) { > > > > > > > > > > >> 1640 runstate_set(RUN_STATE_PRELAUNCH); > > > > > > > > > > >> 1641 } > > > > > > > > > > >> 1642 } > > > > > > > > > > >> ... > > > > > > > > > > >> > > > > > > > > > > >> I already sent a patch for this kind of problem (in > > > > > > > > > > >> current Juan pull > > > > > > > > > > >> request): > > > > > > > > > > >> > > > > > > > > > > >> "runstate: ignore finishmigrate -> prelaunch transition" > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > IIUC your patch avoids an invalid 'prelaunch' -> > > > > > > > > > > > 'postmigrate' runstate > > > > > > > > > > > transition that can happen if the migration thread sets > > > > > > > > > > > the runstate to > > > > > > > > > > > 'finishmigrate' when pause_all_vcpus() releases the main > > > > > > > > > > > loop mutex. > > > > > > > > > > > > > > > > > > > > > > ie. symptom of the problem is QEMU aborting, correct ? > > > > > > > > > > > The issue I'm > > > > > > > > > > > trying to fix is a guest breakage caused by a discrepancy > > > > > > > > > > > between > > > > > > > > > > > QEMU and the guest after migration has succeeded. > > > > > > > > > > > > > > > > > > > > > >> but I don't know if it could fix this one. > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > I don't think so and your patch kinda illustrates it. If > > > > > > > > > > > the runstate > > > > > > > > > > > is 'finishmigrate' when returning from pause_all_vcpus(), > > > > > > > > > > > this means > > > > > > > > > > > that state was sent to the destination before we could > > > > > > > > > > > actually reset > > > > > > > > > > > the machine. > > > > > > > > > > > > > > > > > > > > Yes, you're right. > > > > > > > > > > > > > > > > > > > > But the question behind my comment was: is it expected to > > > > > > > > > > have a pending > > > > > > > > > > reset while we are migrating? > > > > > > > > > > > > > > > > > > > > > > > > > > > > Nothing prevents qemu_system_reset_request() to be called > > > > > > > > > when migration > > > > > > > > > is active. > > > > > > > > > > > > > > > > > > > Perhaps H_CAS can return H_BUSY and wait the end of the > > > > > > > > > > migration and > > > > > > > > > > then be fully executed on destination? > > > > > > > > > > > > > > > > > > > > > > > > > > > > And so we would need to teach SLOF to try H_CAS again until > > > > > > > > > it stops > > > > > > > > > returning H_BUSY ? It seems safer to migrate the CAS reboot > > > > > > > > > flag IMHO. > > > > > > > > > > > > > > > > > > > > > > > > > Ok I've tried that with a patched SLOF that sleeps 500ms and > > > > > > > > tries CAS > > > > > > > > again if H_BUSY was returned. It fixes the issue but it looks a > > > > > > > > bit > > > > > > > > ugly because of the polling with an arbitrary timeout in > > > > > > > > SLOF... I'm > > > > > > > > not very comfortable either with calling migration_is_active() > > > > > > > > from > > > > > > > > the CAS code in QEMU. > > > > > > > > > > > > > > > > David, > > > > > > > > > > > > > > > > Any suggestion ? > > > > > > > > > > > > > > Yeah, I think looping in SLOF is a worse idea than migrating the > > > > > > > cas_reboot flag. > > > > > > > > > > > > > > But.. a better solution still might be to just remove the > > > > > > > remaining > > > > > > > causes for CAS reboot entirely. CAS reboots pretty much suck when > > > > > > > they happen, anyway. > > > > > > > > > > > > > > > > > > > I Agree. > > > > > > > > > > > > > With the irq changeover condition removed, I think the remaining > > > > > > > causes are more theoretical than practical situations at this > > > > > > > point. > > > > > > > > > > > > > > > > > > > FWIW, hotpluggging a PCI device before CAS result in a hung guest > > > > > > (not yet > > > > > > investigated the details). > > > > > > > > > > commit 10f12e6450407b18b4d5a6b50d3852dcfd7fff75 > > > > > Author: Daniel Henrique Barboza <[email protected]> > > > > > Date: Wed Aug 30 15:21:41 2017 -0300 > > > > > > > > > > hw/ppc: CAS reset on early device hotplug > > > > > > > > > > I'll have a look to see what can be done here. > > > > > > > > Ah.. yes, that one might be a bit tricky. > > > > > > > > > > So far it seems to be related to SLOF not being able to create > > > new nodes in the DT when parsing the FDT returned by CAS. SLOF > > > stops the parsing and returns an error. The guest ends up with > > > a broken DT and eventually hangs later (in my case the kernel > > > believes it is going to do hash while radix was negotiated with > > > QEMU). I need to dig some more. > > > > Uh... I don't think this is right. I'm pretty sure SLOF *does* create > > new nodes when parsing the CAS FDT, it will need to for > > "ibm,dynamic-reconfiguration-memory" at least. > > > > It can create "memory@" or "ibm,dynamic-reconfiguration-memory" nodes but > explicitly rejects all others.
Huh. Well that's certainly not correct now that we're doing a full
tree rebuild.
> > I've done some looking and I think the actual reasons here are a bit
> > more complicated (but also possibly easier to handle).
> >
> > 1. We can't send hotplug events to the guest until after CAS,
> > because before then we don't know if it can process them, or if
> > it can, which interrupt it uses for them.
> >
> > 2. Queueing hotplug events across CAS for delivery afterwards
> > introduces other complications
> >
> > 3. We need to make sure that each device appears exactly once in
> > either the initial device tree that the guest OS sees, *or* in a
> > hotplug event, not both or neither.
> >
> > Now that we rebuild the DT at CAS time, I think we mightd be able toy
> > handle this by converting such devices to "cold plugged" at CAS time
> > (similarly to what we already do at reset). Since they're in the
> > CAS-time DT which is what the OS will consume, cold plugged is
> > effectively how the OS will see them.
> >
>
> I have tried hacking around to achieve that. Basically calling
> spapr_drc_reset() for all DRCs for which spapr_drc_needed()
> returns true.
>
> > A remaining problem is that new PCI devices won't get BARs assigned by
> > SLOF in this scenario. We'll probably get away with it because of the
> > linux,pci-probe-only property, but I don't know we want to rely on
>
> We currently only expose this property for pseries-4.2 and newer
> machine types... this could be a problem.
Right. I don't like relying on it even for the newer machines, since
it presumably wouldn't work for FreeBSD or AIX.
> > that. PCI bridges hotplugged introduce further complications, because
> > they won't get enumerated.
Alexey has done some testing and looks like there are already a bunch
of problems with PCI bridges hotplugged during SLOF execution.
> >
> > > > > But I agree the other check is more theoretical:
> > > > >
> > > > > /* capabilities that have been added since CAS-generated guest
> > > > > reset.
> > > > > * if capabilities have since been removed, generate another reset
> > > > > */
> > > > > spapr->cas_reboot = !spapr_ovec_subset(ov5_cas_old,
> > > > > spapr->ov5_cas);
> > > > >
> > > > > Unless changing kernels or tempering with the kernel command line, I
> > > > > don't
> > > > > see how some capabilities could change between the two CAS in
> > > > > practice.
> > > >
> > > > Well, we want to be robust and it's at least theoretically possible
> > > > that the guest will request different things on subsequent reboots.
> > >
> > > Yes but in the latter case a full machine reset occurs and
> >
> > Not necessarily. A guest could ask for something on one CAS cycle,
> > then reject it on another, without doing a full reboot. It'd be a
> > pointless thing for the guest to do, but it's possible.
> >
>
> Ok, I was asking later on if we want to support the scenario of
> multiple CAS without an intermediate full reboot. I now have the
> answer :)
Well, "want" might be a bit strong. PAPR allows for the possibility
so we're trying to support it at this stage. Obviously this could
happen relatively easily with the hotplug-before-CAS cause.
> > > spapr->ov5_cas gets cleared, ie. spapr_ovec_subset() returns
> > > true in the check above no matter what.
> >
> > Well, also it could happen if the guest rejects something we put in
> > the initial value of ov5_cas (which is populated from spapr->ov5, so
> > it's not entirely empty).
> >
>
> AFAICT the initial value of ov5_cas after a full machine reset is
> all zeroes until CAS does:
>
> /* full range of negotiated ov5 capabilities */
> spapr_ovec_intersect(spapr->ov5_cas, spapr->ov5, ov5_guest);
>
> which is done between:
>
> ov5_cas_old = spapr_ovec_clone(spapr->ov5_cas);
>
> and
>
> spapr->cas_reboot = !spapr_ovec_subset(ov5_cas_old, spapr->ov5_cas);
>
> So I don't quite understand how ov5_cas_old, ie. spapr->ov5_cas at the
> time the guest calls CAS, could have an "initial value not entirely
> empty"... This can only happen if the guest calls CAS several times
> without doing a full reboot in between. My initial thought was to
> refuse this scenario and fail any subsequent CAS attempt made by
> the guest before a full reboot.
Oh, sorry, my mistake. I say the line:
spapr->ov5_cas = spapr_ovec_clone(spapr->ov5);
but only now realized that only happens in the qtest case. Ok, yes,
looks like this could only happen in the case of multiple CAS reboots
in a row.
>
> > > > However I believe that the original rationale for this check was that
> > > > while we could add things to the device tree for added capabilities,
> > > > we didn't have a way to roll back the changes for removed
> > > > capabilities.
> > > >
> > >
> > > IIUC this is specifically for "removed capabilities since last
> > > CAS". This can happen if:
> > > 1) we're already processing a CAS reboot or,
> > > 2) a freshly rebooted guest calls CAS twice without being rebooted
> > > in between.
> > >
> > > Since a freshly booted or rebooted guest can only trigger a CAS
> > > reboot because of a "hotplug-before-CAS", if we manage to get rid
> > > of this limitation, 1) cannot happen anymore.
> > >
> > > The linux kernel seems to be only calling
> > > "ibm,client-architecture-support"
> > > once during early boot so 2) should _never_ happen. Do we care to support
> > > this scenario anyway ?
> >
> > I think you've missed some things in your reasoning. But it doesn't
> > really matter because the full dt rebuilt should handle it anyway. I
> > have a draft patch which removes this cause for CAS reboots.
> >
> > Still grappling with the hotplug-before-CAS case.
> >
>
> Same here actually. I was struggling with SLOF to have it create new nodes
> for hotplugged-before-CAS devices without crashing :-\
>
> I think I'll wait for your patches to arrive :) Please Cc: me.
>
> > > > Now that we fully rebuild the device tree at CAS, I think this test
> > > > can probably just go, although there's some double checking to do.
> > > >
> > >
> > > I tend to agree.
> >
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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