Re: [PATCH 3/3] vhost: use checked versions of VIRTIO_BIT

Andrew Lunn
2025-10-09T12:47:59Z

On Thu, Oct 09, 2025 at 07:24:16AM -0400, Michael S. Tsirkin wrote:
> This adds compile-time checked versions of VIRTIO_BIT that set bits in
> low and high qword, respectively. Will prevent confusion when people
> set bits in the wrong qword.
>
> Cc: "Paolo Abeni" <pabeni@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> drivers/vhost/net.c | 4 ++--
> include/linux/virtio_features.h | 9 +++++++++
> 2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 43d51fb1f8ea..8b98e1a8baaa 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -76,8 +76,8 @@ static const u64 vhost_net_features[VIRTIO_FEATURES_QWORDS] = {
> (1ULL << VIRTIO_F_ACCESS_PLATFORM) |
> (1ULL << VIRTIO_F_RING_RESET) |
> (1ULL << VIRTIO_F_IN_ORDER),
> - VIRTIO_BIT(VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) |
> - VIRTIO_BIT(VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO),
> + VIRTIO_BIT_HI(VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) |
> + VIRTIO_BIT_HI(VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO),

How any bits in vhost_net_features are currently in use? How likely is
it to go from 2x 64bit words to 3x 64 bit words? Rather than _LO, _HI,
would _1ST, _2ND be better leaving it open for _3RD?

I would also be tempted to rename these macros to include _LO_ and
_HI_ in them. VIRTIO_BIT_HI(VIRTIO_LO_F_IN_ORDER) is more likely to be
spotted as wrong this way.

An alternative would be to convert to a linux bitmap, which is
arbitrary length so you just use bit number and leave the
implementation to map that to the correct offset in the underlying
data structure.

Andrew