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Re: [Qemu-ppc] [PATCH for-2.11 v2] hw/ppc: CAS reset on early device hot

From: Daniel Henrique Barboza
Subject: Re: [Qemu-ppc] [PATCH for-2.11 v2] hw/ppc: CAS reset on early device hotplug
Date: Tue, 29 Aug 2017 17:54:28 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 08/29/2017 04:23 AM, David Gibson wrote:
On Fri, Aug 25, 2017 at 06:11:18PM -0300, Daniel Henrique Barboza wrote:
- rebased with ppc-for-2.11
- function 'spapr_cas_completed' dropped
- function 'spapr_drc_needed' made public and it's now used inside
- 'spapr_drc_needed' was changed to support the migration of logical
   DRCs with devs attached in UNUSED state
- new function: 'spapr_clear_pending_events'. This function is used
   inside ppc_spapr_reset to reset the pending_events QTAILQ
Thanks for the followup, unfortunately there is still an important bug
left, see comments on the patch itself.

At a higher level, though, looking at the event reset code made me
think of a possible even simpler solution to this problem.

The queue of events (both hotplug and epow) is already in a simple
internal form that's independent of the two delivery mechanisms.  The
only difference is what event source triggers the interrupt.  This
explains why an extra hotplug event after the CAS "unstuck" the queue.

AFAICT, a spurious interrupts here should be harmless - the kernel
will just check the queue and find nothing there.

So, it should be sufficient to, after CAS, pulse the hotplug queue
interrupt if the hotplug queue is negotiated.

This is something I've tried in my first attempts at this problem, before
sending the first patch in which I blocked hotplug before CAS. Back then,
the problem was that the kernel panics with sig 11 (acess of bad area) when
receiving the pulse after CAS.

I've investigated it a bit today and it seems that it still the case. Firing an IRQ right after CAS breaks the kernel. In fact, if you time a regular CPU hotplug right after CAS you'll get the same sig 11 kernel ooops. It looks like there is a time window after
CAS that the kernel can't handle the hotplug process and pulsing the hotplug
queue in this window breaks the guest. I've tried some hacks such as pulsing the queue in the first 'event_scan' call made by the guest, but apparently it is still too early.

I've sent an email to the linuxppc-dev mailing list talking about this behavior and asking if there is a reliable way to know when we can safely pulse the hotplug queue. Meanwhile, I'll keep working in the v3 respin of this patch in case this
solution of pulsing the hotplug queue ends up being not feasible.



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