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Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path()
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path() |
Date: |
Tue, 15 Jun 2010 13:42:30 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) |
Jan Kiszka <address@hidden> writes:
> Markus Armbruster wrote:
>> Paul Brook <address@hidden> writes:
>>
>>> Alex Williamson <address@hidden> 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.
Auto-generated IDs would become part of the ABI. Really so bad that
it's "no option"? Mind, device ID becomes mandatory *only* for devices
that don't have a useful bus address. We could even waive the ID
requirement for the first device of a kind, i.e. require ID if and only
if it's needed to disambiguate.
>> 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.
I'm not arguing against the use of driver names at all.
>> 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.
It's not either/or. Addressing by ID continues to work. Addressing by
bus/driver-name continues to work. We merely add addressing by
bus/@bus-address.
>> 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.
I object to <driver>@<bus-address>, because the <driver> part carries no
information.
Not the case for <driver>.<instance-no>. We still need a suitable
definition of <instance-no>. Possible definitions:
* n-th creation of a <driver> device. Drawbacks: depends on creation
order. Relatively hard to maintain across migration.
* n-th instance of a <driver> device. Drawback: changes on unplug.
Good enough for interactive use, but it doesn't provide a stable
device name.
When counting <driver> devices either way, we can count per bus or
globally. I prefer per bus.
None of the above instance numbers are nearly as neat as bus addresses.
>> 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
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), (continued)
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Markus Armbruster, 2010/06/16
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Alex Williamson, 2010/06/17
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Alex Williamson, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Markus Armbruster, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Alex Williamson, 2010/06/18
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(),
Markus Armbruster <=
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/15
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Markus Armbruster, 2010/06/15
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Paul Brook, 2010/06/15
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Paul Brook, 2010/06/15
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/15
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Markus Armbruster, 2010/06/16
- Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path(), Jan Kiszka, 2010/06/14
[Qemu-devel] [RFC PATCH 2/5] savevm: Add DeviceState param, Alex Williamson, 2010/06/14
[Qemu-devel] [RFC PATCH 3/5] savevm: Make use of the new DeviceState param, Alex Williamson, 2010/06/14
[Qemu-devel] [RFC PATCH 4/5] eepro100: Add a dev field to eeprom new/free functions, Alex Williamson, 2010/06/14