Re: RFC qdev path semantics

Alex Williamson
2010-06-17T21:43:43Z

I'm a little bit lost at how to implement something to print these
semantics, but a couple comments below...

On Wed, 2010-06-16 at 11:46 +0200, Markus Armbruster wrote:
> A number of changes to qdev paths have been proposed in various threads.
> It's becoming harder to keep track of them, so let me sum them up in one
> place. Please correct me if I misrepresent your ideas.
>
> I'm going to describe the current state of things, and the proposed
> changes (marked with ###).
>
>
> The device tree has the main system bus as root. A child of a bus is a
> device. A child of a device is a bus.
>
> A qdev path consists of qdev path components separated by '/'. It
> resolves to a node in the device tree, either bus or device.
>
> The qdev path "/" resolves to the root, i.e. the main system bus.
>
> The qdev path IDENT, where IDENT is an identifier, resolves to the
> device whose id is IDENT.
>
> If PATH resolves to device DEV, and a child of DEV has the name IDENT,
> then we resolve to that bus.
>
> Bus names are chosen by the system as follows:
>
> * If the driver of the parent device model provides a name, use that.
>
> * Else, if the parent device has id ID, use ID.NUM, where NUM is the bus
> number, counting from zero in creation order.
>
> * Else, use TYPE.NUM, where TYPE is derived from the bus type, and NUM
> is the bus number, as above.
>
> ### Paul proposes to drop ID.NUM.
>
> ### Paul proposes to either drop TYPE.NUM (and require drivers to
> provide bus names), or make NUM count separately for each bus type.

I agree, instance numbers are not stable. Would it be reasonable to
outlaw instance numbers of any kind for devices that are children of
buses with BusState.allow_hotplug == 1?

> If PATH resolves to bus BUS, then we resolve PATH/IDENT as follows:
>
> * If a child of BUS has id IDENT, then we resolve to that device.
>
> ### Jan proposes to drop this rule.
>
> * Else, if a child of BUS has a driver with name IDENT, then we resolve
> to that device.
>
> If more than one exist, resolve to the first one. This assumes
> children are ordered. Order is the same as in "info qtree".
> Currently, it's reverse creation order.
>
> This is *not* a stable address.
>
> * Else, if a child of BUS has a driver with alias name IDENT, then we
> resolve to that device.
>
> If more than one exist, resolve to the first one. This assumes
> children are ordered. Order is the same as in "info qtree".
> Currently, it's reverse creation order.
>
> This is *not* a stable address.
>
> ### I propose: we resolve PATH/@BUS-ADDR to the child of BUS with bus
> address BUS-ADDR, if devices on this type of bus have bus addresses.
> The format of BUS-ADDR depends on the bus.
>
> ### Paul proposes to require all buses to define bus addresses. Make
> one up if necessary.

That seems arbitrary and prone to breakage. How do we handle a subtle
change in device instantiation order and still allow migration? If by
code change or command line ordering my frobnitz moves from:

/i440FX-pcihost/pci.0/PIIX3/@01.0/isa.0/0

to

/i440FX-pcihost/pci.0/PIIX3/@01.0/isa.0/1

and it has a vmstate or ramblock associated with it, how do we match
those up?

> ### Jan proposes: we resolve PATH/IDENT.NUM as follows:

This isn't stable. Same as above. I don't think we can allow this on
buses support hotplug.

> * If a child of BUS has a driver with name IDENT and an instance
> number NUM, then we resolve to that device.
>
> Need a suitable definition of "instance number".
>
> Jan proposes to number devices with the same driver on the same
> bus. This assumes children are ordered, see above.
>
> This is *not* a stable address if the bus supports hot-plug.
>
> I propose to us bus-address as instance number. Works best
> together with Paul's proposal to define bus addresses. Syntax
> IDENT@BUS-ADDR makes more sense then.
>
> * Else, same with driver alias name.
>
> ### Here's a possible synthesis of the above three proposals: require
> bus addresses, and permit any of
>
> PATH/IDENT
> PATH/@BUS-ADDR
> PATH/IDENT@BUS-ADDR
>
> PATH/IDENT can't address instances that don't come first.
>
> IDENT in PATH/IDENT@BUS-ADDR is redundant. Therefore, it can't be
> the canonical qdev path. That's fine, PATH/@BUS-ADDR serves.

I can live with PATH/@BUS-ADDR if it's still felt that
PATH/IDENT@BUS-ADDR isn't canonical. What that means is that I'll
probably code up vmstate and ramblocks to append IDENT themselves to
keep all the goodness of having per PATH/IDENT namespaces. Do we want
to settle on a standard end of path delineation? Should I use

PATH/@BUS-ADDR$IDENT.foo

?

Alex

> If the above rules resolve PATH to a device in a context where we expect
> a bus, and the device has exactly one bus, resolve it to that bus
> instead.
>
> ### Jan and I propose to drop this rule.