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Re: [qemu-s390x] [Qemu-devel] [PATCH v3 1/2] s390x/pci: Introduce unplug

From: Pierre Morel
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v3 1/2] s390x/pci: Introduce unplug requests and split unplug handler
Date: Tue, 29 Jan 2019 17:54:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 29/01/2019 16:14, David Hildenbrand wrote:
On 29.01.19 14:31, Pierre Morel wrote:
On 21/01/2019 14:42, David Hildenbrand wrote:
PCI on s390x is really weird and how it was modeled in QEMU might not have
been the right choice. Anyhow, right now it is the case that:
- Hotplugging a PCI device will silently create a zPCI device
    (if none is provided)
- Hotunplugging a zPCI device will unplug the PCI device (if any)
- Hotunplugging a PCI device will unplug also the zPCI device
As far as I can see, we can no longer change this behavior. But we
should fix it.

So unplugging PCI first will deny the guest any possibility to smoothly
relinquish a device.

Is it possible the other way around?

Maybe, but it does not really matter. We unplug both devices
synchronously, without the guest recognizing the order. We always have
the unplug request first that notifies the guest. When we get an ACK
from the guest, we can unpplug both devices in any order.

(and if we want to change the order, we should do it in a separate
patch, this patch does not change the order, just refactors the code)

If it is done atomically, then I have no objection.


Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany

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