Re: [PATCH net-next 2/8] virtio_pci_modern: allow setting configuring extended features

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 29, 2025 at 01:07:30PM +0200, Paolo Abeni wrote:
> On 5/29/25 4:22 AM, Jason Wang wrote:
> > On Thu, May 29, 2025 at 12:02 AM Paolo Abeni <pabeni@redhat.com> wrote:
> >> On 5/27/25 5:04 AM, Jason Wang wrote:
> >>> On Mon, May 26, 2025 at 6:53 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >>>> On 5/26/25 2:49 AM, Jason Wang wrote:
> >>>>> On Wed, May 21, 2025 at 6:33 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >>>>>>
> >>>>>> The virtio specifications allows for up to 128 bits for the
> >>>>>> device features. Soon we are going to use some of the 'extended'
> >>>>>> bits features (above 64) for the virtio_net driver.
> >>>>>>
> >>>>>> Extend the virtio pci modern driver to support configuring the full
> >>>>>> virtio features range, replacing the unrolled loops reading and
> >>>>>> writing the features space with explicit one bounded to the actual
> >>>>>> features space size in word.
> >>>>>>
> >>>>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> >>>>>> ---
> >>>>>>  drivers/virtio/virtio_pci_modern_dev.c | 39 +++++++++++++++++---------
> >>>>>>  1 file changed, 25 insertions(+), 14 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
> >>>>>> index 1d34655f6b658..e3025b6fa8540 100644
> >>>>>> --- a/drivers/virtio/virtio_pci_modern_dev.c
> >>>>>> +++ b/drivers/virtio/virtio_pci_modern_dev.c
> >>>>>> @@ -396,12 +396,16 @@ EXPORT_SYMBOL_GPL(vp_modern_remove);
> >>>>>>  virtio_features_t vp_modern_get_features(struct virtio_pci_modern_device *mdev)
> >>>>>>  {
> >>>>>>         struct virtio_pci_common_cfg __iomem *cfg = mdev->common;
> >>>>>> -       virtio_features_t features;
> >>>>>> +       virtio_features_t features = 0;
> >>>>>> +       int i;
> >>>>>>
> >>>>>> -       vp_iowrite32(0, &cfg->device_feature_select);
> >>>>>> -       features = vp_ioread32(&cfg->device_feature);
> >>>>>> -       vp_iowrite32(1, &cfg->device_feature_select);
> >>>>>> -       features |= ((u64)vp_ioread32(&cfg->device_feature) << 32);
> >>>>>> +       for (i = 0; i < VIRTIO_FEATURES_WORDS; i++) {
> >>>>>> +               virtio_features_t cur;
> >>>>>> +
> >>>>>> +               vp_iowrite32(i, &cfg->device_feature_select);
> >>>>>> +               cur = vp_ioread32(&cfg->device_feature);
> >>>>>> +               features |= cur << (32 * i);
> >>>>>> +       }
> >>>>>
> >>>>> No matter if we decide to go with 128bit or not. I think at the lower
> >>>>> layer like this, it's time to allow arbitrary length of the features
> >>>>> as the spec supports.
> >>>>
> >>>> Is that useful if the vhost interface is not going to support it?
> >>>
> >>> I think so, as there are hardware virtio devices that can benefit from this.
> >>
> >> Let me look at the question from another perspective. Let's suppose that
> >> the virtio device supports an arbitrary wide features space, and the
> >> uAPI allows passing to/from the kernel an arbitrary high number of features.
> >>
> >> How could the kernel stop the above loop? AFAICS the virtio spec does
> >> not define any way to detect the end of the features space. An arbitrary
> >> bound is actually needed.
> > 
> > I think this is a good question ad we have something that could work:
> > 
> > 1) current driver has drv->feature_table_size, so the driver knows
> > it's meaningless to read above the size
> > 
> > and
> > 
> > 2) we can extend the spec, e.g add a transport specific field to let
> > the driver to know the feature size
> 
> So I guess we can postpone any additional change here until we have some
> spec in place, right?
> 
> /P

Agree on this.