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Re: [qemu-s390x] [Qemu-devel] [PATCH v3 2/2] s390x/pci: Unplug remaining

From: Pierre Morel
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v3 2/2] s390x/pci: Unplug remaining devices on pcihost reset
Date: Tue, 29 Jan 2019 14:50:56 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 29/01/2019 11:24, David Hildenbrand wrote:
I'm wondering what the architecture says regarding those events -- can
someone with access to the documentation comment?

Ping. Any comments from the IBM folks?


Sorry to have wait so long.
At least Collin was faster.

So the idea here is that if we have a PCI device that is the process of
being deconfigured and we are also in the middle of a reset, then let's
accelerate deconfiguring of the PCI device during the reset. Makes sense.

to me too.
However, how do we ensure that the guest got time to respond to the first deconfigure request?


The callback function will deconfigure the the device and put it into
standby mode. However, a PCI device should only go into standby from the
*disabled state* (which it could already be in due to the unplug
sequence), or from a *permanent error state* (something we should
hopefully never see -- this means something went seriously wrong with
the device).

Not completely exact, the CHSC event 0x304, on the initiative of the host, force the "deconfigure state" from "configure state" generally, whatever sub-state it has (enabled/disabled/error...).

Right, this should already have been checked before setting up the timer.

Apropos timer, do we need a timer or wouldn't it be better to use a delay / a timer + condition?

AFAIU we get out of the unplug without waiting for any answer from the guest and we surely get the timer triggering after the reset has been done.
That seems bad.

Two things I'm concerned about:


What I would suggest is adding a check for the pbdev->state for
ZPCI_FS_DISABLED || ZPCI_FS_PERMANENT_ERROR. If it is in either of these
states, then we're safe to deconfigure and put into standby. If the

We setup a timer if !ZPCI_FS_STANDBY and !ZPCI_FS_RESERVED.

So for

We setup a timer and simply go ahead and unplug the device when the
timer expires ("forced unplug").

I agree only for ZPCI_FS_ENABLED why do we need to be smooth for other states? ZPCI_FS_DISABLED may be a candidate even I do not see the interrest but other states of the device should issue a HP_EVENT_DECONFIGURE_REQUEST and we do not need a timer (or any delay)

Changing that behavior might be more invasive. Simply not unplugging in
s390_pcihost_timer_cb() on some of these states would mean that somebody
issued a request and that requests is simply lost/ignored. Not sure if
that is what we want. I think we need separate patches to change
something like that. Especially

1. What happens if the device was in ZPCI_FS_DISABLED, the guest ignores
the unplug request and moves the device to ZPCI_FS_ENABLED before the
timer expires? These are corner cases to consider.

+1, we must ensure to do the work inside the unplug CB.

2. Do we need a timer at all? Now that Patch #1 introduces
unplug_requests, we are free to ignore requests from the user if the
guest is not reacting. I would really favor getting rid of the timer
completely. Was there a special reason why this was introduced?

Yes, to let a chance to the guest to smoothly relinquish the device.
(for example sync/clean the disk)
However I do not think it is right implemented.

No other architecture (e.g. ACPI) uses such a timer. They use a simple
flag to remember if a request is pending. I would really favor going
into that direction.

I am not sure that the Intel architecture is a good example. :)

AFAIU they do not wait for the guest to have relinquish the device.
Or do they?
How long do they wait?

@Christian do you think we need this "force remove after 30 seconds"
timer thingy?

device is still in another state (such as enabled or blocked, etc) then
we should allow the timer to resume and give the device some more time
before forcing an unplug. It's also probably not a good idea to try and
deconfigure a device that might already be deconfigured (e.g. if it's
already in standby or reserved state). That might not happen though, but
it's good to cover our bases.

A side thought: In addition to checking the states, what would happen if
you forced the timer to 0? Would the callback get called? Would that
just accelerate the already-in-progress unplug sequence?

I guess so, but then action will be called asynchronously from the main
loop when timers are run. Calling it synchronously during the reset
makes in my point of view more sense.

To me too, (and during hot unplug too) but it has a sense only if we wait some time between the demand to relinquish the device and the rip off of the device.


Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany

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