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

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Message-ID: <f0a36685-45d0-4c4a-a256-74f3d4a31bd5@redhat.com>
Date: Thu, 29 May 2025 13:07:30 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Wang <jasowang@...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 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