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Message-ID: <20250529102840-mutt-send-email-mst@kernel.org>
Date: Thu, 29 May 2025 10:28:51 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, 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>,
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 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@...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
>
> So I guess we can postpone any additional change here until we have some
> spec in place, right?
>
> /P
Agree on this.
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