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

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Message-ID: <CACGkMEtZZbN8vj-V-PSwAmQKCP=gDN5sDz4TOXcOhNXGPLp_yQ@mail.gmail.com>
Date: Tue, 27 May 2025 11:04:49 +0800
From: Jason Wang <jasowang@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Eugenio Pérez <eperezma@...hat.com>
Subject: Re: [PATCH net-next 2/8] virtio_pci_modern: allow setting configuring
 extended features

On Mon, May 26, 2025 at 6:53 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 5/26/25 2:49 AM, Jason Wang wrote:
> > On Wed, May 21, 2025 at 6:33 PM Paolo Abeni <pabeni@...hat.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@...hat.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.

>
> Note that the above code is independent from the feature-space. Defining
> larger value of VIRTIO_FEATURES_WORDS it will deal with larger number of
> features.
>
> /P
>

Thanks