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

[Jan Kiszka <jan.kiszka@siemens.com>]

Markus Armbruster wrote:
> 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.

No option due to auto-created devices. And auto-generating IDs would
just create usability issues.

> 
> 
> 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!

>From user POV, driver names are very handly to address a device
intuitively - except for the case you have tones of devices on the same
bus that are handled by the same driver. For that case we need to
augment the device name with a useful per-bus ID, derived from the bus
address where available, otherwise based on instance numbers.

> 
> 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".

And I prefer "device_del [.../]pci.0/e1000". Otherwise you need to dump
the bus first before you can identify which device you want to remove.

> 
> 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.

I would prefer <driver>[@<bus-address>|.<instance-no>]. The former is
set for buses that implement some to-be-defined device addressing
service, the latter is the default on buses where that service is not
available.

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

Agreed, we should find one solution for all use cases.

Jan

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