Re: [PATCH RFC 0/2] virtio-ccw: allow to disable legacy virtio

[Cornelia Huck <cohuck@redhat.com>]

On Thu,  4 Mar 2021 14:27:13 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

Just found this old series sitting there... does anyone have an opinion
on the general approach?

> Unlike virtio-pci, virtio-ccw is currently always a transitional
> driver (i.e. it always includes support for legacy devices.) The
> differences between legacy and virtio-1+ virtio-ccw devices are not
> that big (the most interesting things are in common virtio code
> anyway.)
> 
> It might be beneficial to make support for legacy virtio generally
> configurable, in case we want to remove it completely in a future
> where we all have flying cars. As a prereq, we need to make it
> configurable for virtio-ccw.
> 
> Patch 1 introduces a parameter; now that I look at it, it's probably
> not that useful (not even for testing), so I'm inclined to drop it
> again.
> 
> Patch 2 adds a new config symbol for generic legacy virtio support,
> which currently does not do anything but being selected by the
> legacy options for virtio-pci and virtio-ccw. A virtio-ccw driver
> without legacy support will require a revision of 1 or higher to
> be supported by the device.
> 
> A virtio-ccw driver with legacy turned off works well for me with
> transitional devices and fails onlining gracefully for legacy devices
> (max_revision=0 in QEMU).
> 
> (I also have some code that allows to make devices non-transitional
> in QEMU, but I haven't yet found time to polish the patches.)
> 
> Cornelia Huck (2):
>   virtio/s390: add parameter for minimum revision
>   virtio/s390: make legacy support configurable
> 
>  arch/s390/Kconfig                       |  11 ++
>  drivers/s390/virtio/Makefile            |   1 +
>  drivers/s390/virtio/virtio_ccw.c        | 179 ++++++++----------------
>  drivers/s390/virtio/virtio_ccw_common.h | 113 +++++++++++++++
>  drivers/s390/virtio/virtio_ccw_legacy.c | 138 ++++++++++++++++++
>  drivers/virtio/Kconfig                  |   8 ++
>  6 files changed, 330 insertions(+), 120 deletions(-)
>  create mode 100644 drivers/s390/virtio/virtio_ccw_common.h
>  create mode 100644 drivers/s390/virtio/virtio_ccw_legacy.c
> 
> 
> base-commit: cf6acb8bdb1d829b85a4daa2944bf9e71c93f4b9