Re: [Qemu-devel] [PATCH] virtio: Provide version-specific variants of virtio PCI devices

[Eduardo Habkost]

On Wed, Oct 17, 2018 at 10:22:37AM +0100, Stefan Hajnoczi wrote:
> On Fri, Oct 12, 2018 at 11:54:35PM -0300, Eduardo Habkost wrote:
> > This patch adds separate device types for each of those virtio
> > device flavors:
> > 
> > - virtio-*-pci: the existing multi-purpose device types
> >   - Configurable using `disable-legacy` and `disable-modern`
> >     properties
> >   - Legacy driver support is automatically enabled/disabled
> >     depending on the bus where it is plugged
> >   - Supports Conventional PCI and PCI Express buses
> >     (but Conventional PCI is incompatible with
> >     disable-legacy=off)
> >   - Changes PCI vendor/device IDs at runtime
> > - virtio-*-pci-0.9: legacy virtio device
> >   - Supports Conventional PCI buses only, because
> >     it has a PIO BAR
> > - virtio-*-pci-1.0-transitional: virtio-1.0 device supporting legacy drivers
> >   - Supports Conventional PCI buses only, because
> >     it has a PIO BAR
> > - virtio-*-pci-1.0: modern-only
> >   - Supports both Conventional PCI and PCI Express buses
> 
> If new device types are being created, is it time to decouple the VIRTIO
> PCI transport from the actual VIRTIO device (blk, net, scsi) like we
> already have with virtio-mmio?
> 
>   -device virtio-pci-1.0,id=virtio-pci-0
>   -device virtio-blk,bus=virtio-pci-0,drive=drive0,serial=mydisk
> 
> That way we avoid lots of boilerplate code and an explosion of new
> device types.

I don't think that would work.  This would in turn make
virtio-pci-1.0 a chameleon device that changes PCI ID depending
on the backend device plugged to it.

Also, PCI IDs are not the only information contained inside the
virtio-pci subclasses.  See all the PCI-specific +
device-type-specific knowledge encoded inside the class_init and
instance_init functions of each virtio-pci subclass.

-- 
Eduardo