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Re: [Qemu-devel] [PATCH v2 4/6] s390x/pci: Introduce unplug requests and

From: Collin Walling
Subject: Re: [Qemu-devel] [PATCH v2 4/6] s390x/pci: Introduce unplug requests and split unplug handler
Date: Thu, 31 Jan 2019 15:40:17 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 1/30/19 10:57 AM, 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.

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 devices that are not
    hot removable. (check performed in qdev_unplug()) - bad.
2. 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.

The redirect part is a little hairy, as the user is allowed to trigger
unplug either via the PCI or the zPCI device. So redirect always to the
PCI unplug request handler first and remember if that check has been
performed in the zPCI device. Redirect then to the zPCI device unplug
request handler to perform the magic. Remembering that we already
checked the PCI device breaks the redirect loop.

Signed-off-by: David Hildenbrand <address@hidden>

Thanks for clearing up some of the areas I was having troubles following
during the last post. I think this looks a lot better than what we had
before. Patches 4, 5, and 6 make the code a lot easier to understand
what is going on here.

I assume you were able to squeeze in an easy test? At least making sure
we can still hot unplug a device from a guest successfully?

From the code perspective:

Reviewed-by: Collin Walling <address@hidden>

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