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

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Message-ID: <CACGkMEuBrzozRYqrgu8pM-+Ke2-NhCbFRHr8NeVpP15Qo0RZGg@mail.gmail.com>
Date: Thu, 29 May 2025 10:22:40 +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 Thu, May 29, 2025 at 12:02 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 5/27/25 5:04 AM, Jason Wang wrote:
> > 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.
>
> 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

>
> If 128 looks too low (why?) it can be raised to say 256 (why?). But
> AFAICS the only visible effect would be slower configuration due to
> larger number of unneeded I/O operations.

See above.

>
> /P
>

Thanks