Re: [PATCH] hw/block/nvme: re-enable NVMe PCI hotplug

[Klaus Jensen]

On May 11 16:54, Hannes Reinecke wrote:
> On 5/11/21 3:37 PM, Klaus Jensen wrote:
> > On May 11 15:12, Hannes Reinecke wrote:
> > > On 5/11/21 2:22 PM, Klaus Jensen wrote:
> [ .. ]
> > > > The hotplug fix looks good - I'll post a series that tries to integrate
> > > > both.
> > > Ta.
> > >
> > > The more I think about it, the more I think we should be looking into
> > > reparenting the namespaces to the subsystem.
> > > That would have the _immediate_ benefit that 'device_del' and
> > > 'device_add' becomes symmetric (ie one doesn't have to do a separate
> > > 'device_add nvme-ns'), as the nvme namespace is not affected by the
> > > hotplug event.
> >
> > I have that working, but I'm struggling with a QEMU API technicality in
> > that I apparently cannot simply move the NvmeBus creation to the
> > nvme-subsys device. For some reason the bus is not available for the
> > nvme-ns devices. That is, if one does something like this:
> >
> >   -device nvme-subsys,...
> >   -device nvme-ns,...
> >
> > Then I get an error that "no 'nvme-bus' bus found for device 'nvme'ns".
> > This is probably just me not grok'ing the qdev well enough, so I'll keep
> > trying to fix that. What works now is to have the regular setup:
> _Normally_ the 'id' of the parent device spans a bus, so the syntax
> should be
>
> -device nvme-subsys,id=subsys1,...
> -device nvme-ns,bus=subsys1,...

Yeah, I know, I just oversimplified the example. This *is* how I wanted 
it to work ;)

> As for the nvme device I would initially expose any namespace from the
> subsystem to the controller; the nvme spec has some concept of 'active'
> or 'inactive' namespaces which would allow us to blank out individual
> namespaces on a per-controller basis, but I fear that's not easy to
> model with qdev and the structure above.

The nvme-ns device already supports the boolean 'detached' parameter to 
support the concept of an inactive namespace.

> >   -device nvme-subsys,...
> >   -device nvme,...
> >   -device nvme-ns,...
> >
> > And the nvme-ns device will then reparent to the NvmeBus on nvme-subsys
> > (which magically now IS available when nvme-ns is realized). This has
> > the same end result, but I really would like that the namespaces could
> > be specified as children of the subsys directly.
> Shudder.
> Automatic reparenting.
> To my understanding from qdev that shouldn't even be possible.
> Please don't.

It's perfectly possible with the API and used to implement stuff like 
failover. We are not changing the parent object, we are changing the 
parent bus. hw/sd does something like this (but does mention that its a 
bit of a hack). In this case I'd say we could argue to get away with it 
as well. 

Allowing the nvme-ns device to be a child of the controller allows the 
initially attached controller of non-shared namespaces to be easily 
expressible. But I agree, the approach is a bit wacky, which is why I 
havnt posted anything yet.

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