Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path()

Markus Armbruster
2010-06-15T08:47:27Z

Paul Brook <paul@codesourcery.com> writes:

> Alex Williamson <alex.williamson@redhat.com> writes:
>
>> On Mon, 2010-06-14 at 08:39 +0200, Markus Armbruster wrote:
>> > Could you explain why you add "identified properties of the immediate
>> > parent bus and device"? They make the result ver much *not* a "dev
>> > path" in the qdev sense...
>>
>> In order to try to get a unique string. Without looking into device
>> properties, two e1000s would both be:
>>
>> /main-system-bus/pci.0/e1000
>> /main-system-bus/pci.0/e1000
>>
>> Which is no better than simply "e1000" and would require us to fall back
>> to instance ids again. The goal here is that anything that makes use of
>> passing a dev when registering a vmstate gets an instance id of zero.
>
> You already got the information you need, you just put it in the wrong place.
> The canonical ID for the device could be its bus address. In practice we'd
> probably want to allow the user to specify it by name, provided these are
> unique. e.g. in the above machine we could accept [...]/virtiio-blk-pci would
> as an aias for [...]:_09.0. Device names have a restricted namespace, so we
> can use an initial prefix to disambiguate a name/label from a bus address.
>
> For busses that don't have a consistent addressing scheme then some sort of
> instance ID is unavoidable. I guess it may be possible to invent something
> based on other device properties (e.g. address of the first IO port/memory
> region).

When that's inconvenient or impossible, we can still punt to user: make
device ID mandatory.

We obviously need a way to unambigously name a device. It's okay to
have multiple names for the same device.

If the device has a device ID, that's an unambigous name.

qdev paths may be ambigous when path components are resolved to driver
names instead of IDs.

Alex proposed to disambiguate by adding "identified properties of the
immediate parent bus and device" to the path component. For PCI, these
are dev.fn. Likewise for any other bus where devices have unambigous
bus address. The driver name carries no information!

For other buses, we need to make something up.

Note that addressing by bus address rather than name is generally
useful, not just in the context of savevm. For instance, I'd appreciate
being able to say something like "device_del pci.0/04.0".

An easy way to get that is to reserve part of the name space for bus
addresses. If the path component starts with a letter, it's an ID or
driver name. If it starts with say '@', it's a bus address in
bus-specific syntax. The bus provides a method to look it up.

That way, we gain a useful feature, and avoid having an savevm-specific
"device path" that isn't recognized anywhere else.