Re: [RFC PATCH v4 15/20] vhost: Shadow virtqueue buffers forwarding

[Markus Armbruster <armbru@redhat.com>]

Eugenio Pérez <eperezma@redhat.com> writes:

> Initial version of shadow virtqueue that actually forward buffers. There
> are no iommu support at the moment, and that will be addressed in future
> patches of this series. Since all vhost-vdpa devices uses forced IOMMU,
> this means that SVQ is not usable at this point of the series on any
> device.
>
> For simplicity it only supports modern devices, that expects vring
> in little endian, with split ring and no event idx or indirect
> descriptors. Support for them will not be added in this series.
>
> It reuses the VirtQueue code for the device part. The driver part is
> based on Linux's virtio_ring driver, but with stripped functionality
> and optimizations so it's easier to review. Later commits add simpler
> ones.
>
> SVQ uses VIRTIO_CONFIG_S_DEVICE_STOPPED to pause the device and
> retrieve its status (next available idx the device was going to
> consume) race-free. It can later reset the device to replace vring
> addresses etc. When SVQ starts qemu can resume consuming the guest's
> driver ring from that state, without notice from the latter.
>
> This status bit VIRTIO_CONFIG_S_DEVICE_STOPPED is currently discussed
> in VirtIO, and is implemented in qemu VirtIO-net devices in previous
> commits.
>
> Removal of _S_DEVICE_STOPPED bit (in other words, resuming the device)
> can be done in the future if an use case arises. At this moment we can
> just rely on reseting the full device.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  qapi/net.json                      |   2 +-
>  hw/virtio/vhost-shadow-virtqueue.c | 237 ++++++++++++++++++++++++++++-
>  hw/virtio/vhost-vdpa.c             | 109 ++++++++++++-
>  3 files changed, 337 insertions(+), 11 deletions(-)
>
> diff --git a/qapi/net.json b/qapi/net.json
> index fe546b0e7c..1f4a55f2c5 100644
> --- a/qapi/net.json
> +++ b/qapi/net.json
> @@ -86,7 +86,7 @@
>  #
>  # @name: the device name of the VirtIO device
>  #
> -# @enable: true to use the alternate shadow VQ notifications
> +# @enable: true to use the alternate shadow VQ buffers fowarding path

Uh, why does the flag change meaning half-way through this series?

>  #
>  # Returns: Error if failure, or 'no error' for success.
>  #

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