Re: [virtio-dev] [PATCH] transitional issues: add new IDs for all devices

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> non-transitional devices should have been able to simply update
> revision ID to make sure legacy drivers are not loaded.
> Unfortunately, mistakes were made:
> - we didn't stress that drivers must check revision ID,
>   and of course there's no easy way for drivers to
>   test this failure path,
>   so older versions of Windows drivers ignored revision
>   (latest vision matches revision correctly)
> - CCW lacks revision ID field
>
> Both facts mean a non-transitional device would need
> a separate mechanism to prevent legacy drivers from
> loading.
> We aren't running out of device IDs yet, so
> let's use up some to resolve this.
>
> I incremented all IDs by 0x100 intentionally -
> for the PCI bindings, this should help remind people they can't
> just stick the Subsystem ID into the low byte of the Device ID.

I don't think this actually works.

If you offer a non-transitional device to a legacy driver, it will
fail anyway (device doesn't work due to missing feature 32).  There's
not much difference between that and it not finding the device.

Or are you anticipating guests with both legacy and transitional
drivers?

Cheers,
Rusty.