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Re: [Qemu-devel] [PATCH 1/2] QMP: Introduce commands doc

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH 1/2] QMP: Introduce commands doc
Date: Mon, 17 May 2010 12:09:53 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

On 05/17/2010 11:27 AM, Markus Armbruster wrote:

A slot is the hotpluggable entity.  Open your computer and you can
actually see them.

QEMU doesn't really know that.

How can that be?  Do we signal hotplug notifications to a function or
to a slot?

Can we hotplug a single function in an already occupied slot?
What I meant to say: we have no concept of "slot" in the higher level
interfaces, we have only bus and device.

If a PCI device has multiple functions, we have a separate qdev device
for each function.  You can't unplug a "slot" (concept doesn't exist in
qdev), only a qdev device.  Naturally, when you unplug a qdev device,
all functions in the same PCI slot need to go.  This happens deep down
in the bowels of ACPI, in piix4_device_hotplug().  qdev is not aware of
this magic relation between the qdev devices for functions in the same

IMO, that's a serious bug. A slot is a user visible entity, both in that devices can only be hotplugged only as slots, not functions, and to determine the maximum number of devices you can add. If the user knows about it, qemu should too.

We can easily represent a slot/device as a qbus with each of the functions as devices attached to it.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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