netdev - Re: [PATCH v4 net-next 1/8] virtio: introduce extended features

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Message-ID: <54923be8-8ab0-4b55-b599-5f20c999e60f@redhat.com>
Date: Wed, 18 Jun 2025 10:31:37 +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>, Yuri Benditovich <yuri.benditovich@...nix.com>,
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: Re: [PATCH v4 net-next 1/8] virtio: introduce extended features

On 6/18/25 3:51 AM, Jason Wang wrote:
> On Wed, Jun 18, 2025 at 12:12 AM 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.
>>
>> Introduce extended features as a fixed size array of u64. To minimize
>> the diffstat allows legacy driver to access the low 64 bits via a
>> transparent union.
>>
>> Introduce an extended get_extended_features configuration callback
>> that devices supporting the extended features range must implement in
>> place of the traditional one.
>>
>> Note that legacy and transport features don't need any change, as
>> they are always in the low 64 bit range.
>>
>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>> ---
>> v3 -> v4:
>>   - moved bit sanity check in virtio_features_*
>>   - replaced BUG_ON with WARN_ON_ONCE
>>   - *_and_not -> _andnot
>>   - short circuit features comparison
>> v2 -> v3:
>>   - uint128_t -> u64[2];
>> v1 -> v2:
>>   - let u64 VIRTIO_BIT() cope with higher bit values
>>   - add .get_features128 instead of changing .get_features signature
>> ---
>>  drivers/virtio/virtio.c         | 43 +++++++++-------
>>  drivers/virtio/virtio_debug.c   | 27 +++++-----
>>  include/linux/virtio.h          |  5 +-
>>  include/linux/virtio_config.h   | 41 +++++++--------
>>  include/linux/virtio_features.h | 88 +++++++++++++++++++++++++++++++++
>>  5 files changed, 151 insertions(+), 53 deletions(-)
>>  create mode 100644 include/linux/virtio_features.h
>>
>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
>> index 95d5d7993e5b..5c48788cdbec 100644
>> --- a/drivers/virtio/virtio.c
>> +++ b/drivers/virtio/virtio.c
>> @@ -53,7 +53,7 @@ static ssize_t features_show(struct device *_d,
>>
>>         /* We actually represent this as a bitstring, as it could be
>>          * arbitrary length in future. */
>> -       for (i = 0; i < sizeof(dev->features)*8; i++)
>> +       for (i = 0; i < VIRTIO_FEATURES_MAX; i++)
>>                 len += sysfs_emit_at(buf, len, "%c",
>>                                __virtio_test_bit(dev, i) ? '1' : '0');
>>         len += sysfs_emit_at(buf, len, "\n");
>> @@ -272,22 +272,22 @@ static int virtio_dev_probe(struct device *_d)
>>         int err, i;
>>         struct virtio_device *dev = dev_to_virtio(_d);
>>         struct virtio_driver *drv = drv_to_virtio(dev->dev.driver);
>> -       u64 device_features;
>> -       u64 driver_features;
>> +       u64 device_features[VIRTIO_FEATURES_DWORDS];
>> +       u64 driver_features[VIRTIO_FEATURES_DWORDS];
>>         u64 driver_features_legacy;
>>
>>         /* We have a driver! */
>>         virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
>>
>>         /* Figure out what features the device supports. */
>> -       device_features = dev->config->get_features(dev);
>> +       virtio_get_features(dev, device_features);
>>
>>         /* Figure out what features the driver supports. */
>> -       driver_features = 0;
>> +       virtio_features_zero(driver_features);
>>         for (i = 0; i < drv->feature_table_size; i++) {
>>                 unsigned int f = drv->feature_table[i];
>> -               BUG_ON(f >= 64);
>> -               driver_features |= (1ULL << f);
>> +               if (!WARN_ON_ONCE(f >= VIRTIO_FEATURES_MAX))
> 
> Nit: Any reason why switching to use WARN_ON_ONCE()?

BUG_ON() are strongly discouraged. Since I touched the nearby code
checkpatch urged me to take action ;)

/P