Re: [Qemu-devel] [PATCH 0/2] virtio: reset all qbuses too when writing to the status field

[Paolo Bonzini <pbonzini@redhat.com>]

Il 12/12/2012 22:27, Michael S. Tsirkin ha scritto:
>>> Maybe it's obvious to you that qdev_reset_all(x)
>>> does a soft reset and what soft reset means
>>> for each bus type
>>
>> We can define soft reset to be *independent* of the bus type.  As you
>> said, you access it with a device register.
> 
> I think qemu has one type of reset ATM which is the hard reset.

A hard reset would kill BARs and configuration space too.
qdev_reset_all doesn't.  Ergo, it is not a hard reset.

But hey, I'm not wed to names.  Let's call it device-level reset and
bus-level reset.  Whatever.

> Any other kind is device specific now.
> The fact that qdev has two APIs which are kind of but not
> exactly the same is a bug not a feature.

BusState reset and DeviceState reset are not the same because they are
triggered differently, one by bus-level functionality (e.g. FLR) and the
other by device-level functionality (e.g. a register).

That's neither a bug nor a feature.  It's just obvious.

> And relying on it in generic virtio is just going to
> confuse things.

It is going to confuse you perhaps, but it will not confuse whoever will
write the next virtio device with a bus.  Who knows, virtio-i2c.

> Further it does not follow that all backends are children
> of the frontend.

Backends are not children of the frontend.  Seriously, this is qdev 101.

virtio-scsi has no backend.  Frontends (disks) are children of
virtio-scsi.  Each backend (host block device) is connected to a child
of virtio-scsi.

virtio-scsi does not need to reset back-ends.  It needs to reset front-ends.

virtio-serial does not know it needs to reset back-ends.  It knows it
needs to reset front-ends.  Each reset of the front-end will also
propagate to a back-end, but virtio-serial need not know that.

> So please just fix the virtio-scsi bug for now and we can
> address the bigger issue if any later.

I'm refusing to fix the bug in virtio-scsi.  It is not a virtio-scsi
bug, as proved by the virtio-serial bus having to do the same things.
Two wrongs do not make a right, and here we have three wrongs already:
virtio-pci, virtio-s390, virtio-serial all reinventing the wheel.

Paolo