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

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Message-ID: <CACGkMEv=XnqKDXCEitEOs-AL1g=H=7WiHEaHrMUN-RfKN1JCRg@mail.gmail.com>
Date: Mon, 26 May 2025 08:49:35 +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 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.

Thanks