From: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-ppc@nongnu.org, qemu-devel@nongnu.org, mdroth@linux.vnet.ibm.com
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH for-2.11 v2] hw/ppc: CAS reset on early device hotplug
Date: Tue, 29 Aug 2017 17:54:28 -0300 [thread overview]
Message-ID: <df49df06-caaf-81f5-7c24-f4324654a262@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170829072310.GJ2578@umbus.fritz.box>
On 08/29/2017 04:23 AM, David Gibson wrote:
> On Fri, Aug 25, 2017 at 06:11:18PM -0300, Daniel Henrique Barboza wrote:
>> v2:
>> - rebased with ppc-for-2.11
>> - function 'spapr_cas_completed' dropped
>> - function 'spapr_drc_needed' made public and it's now used inside
>> 'spapr_hotplugged_dev_before_cas'
>> - '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.
Thanks,
Daniel
next prev parent reply other threads:[~2017-08-29 20:54 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-25 21:11 [Qemu-devel] [PATCH for-2.11 v2] hw/ppc: CAS reset on early device hotplug Daniel Henrique Barboza
2017-08-25 21:11 ` Daniel Henrique Barboza
2017-08-29 6:45 ` David Gibson
2017-08-29 7:23 ` David Gibson
2017-08-29 20:54 ` Daniel Henrique Barboza [this message]
2017-08-30 2:07 ` [Qemu-devel] [Qemu-ppc] " David Gibson
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=df49df06-caaf-81f5-7c24-f4324654a262@linux.vnet.ibm.com \
--to=danielhb@linux.vnet.ibm.com \
--cc=david@gibson.dropbear.id.au \
--cc=mdroth@linux.vnet.ibm.com \
--cc=qemu-devel@nongnu.org \
--cc=qemu-ppc@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.