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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 14:31:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

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.

hum, is it really a problem per se?

Both device types are handled via a single hotplug handler call. This
is problematic for various reasons:
1. Unplugging via the zPCI device allows to unplug PCI bridges as
    checks are not performed - bad.


2. Unplugging via the zPCI device allows to unplug devices that are not
    hot removable. (check performed in qdev_unplug()) - bad.


3. Hotplug handler chains are not possible for the unplug case. In the
    future, the machine might want to override hotplug handlers, to
    process device specific stuff and to then branch off to the actual
    hotplug handler. We need separate hotplug handler calls for both the
    PCI and zPCI device to make this work reliably. All other PCI
    implementations are already prepared to handle this correctly, only
    s390x is missing.


Therefore, introduce the unplug_request handler and properly perform
unplug checks by redirecting to the separate unplug_request handlers.
When finally unplugging, perform two separate hotplug_handler_unplug()
calls, first for the PCI device, followed by the zPCI device. This now
nicely splits unplugging paths for both devices.

hum, PCI device handle the backend, host side, while zPCI handle the front end, guest side. So unplugging PCI first will deny the guest any possibility to smoothly relinquish a device.

Is it possible the other way around?


Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany

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