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

Michael S. Tsirkin
2025-10-09T11:24:23Z

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),
};

enum {
diff --git a/include/linux/virtio_features.h b/include/linux/virtio_features.h
index f41acb035af9..466f7d8ed5ba 100644
--- a/include/linux/virtio_features.h
+++ b/include/linux/virtio_features.h
@@ -9,6 +9,15 @@
#define VIRTIO_FEATURES_DWORDS (VIRTIO_FEATURES_QWORDS * 2)
#define VIRTIO_BIT(b) BIT_ULL((b) & 0x3f)
#define VIRTIO_QWORD(b) ((b) >> 6)
+
+/* Get a given feature bit in a given qword. */
+#define VIRTIO_BIT_QWORD(bit, qword) \
+ (BUILD_BUG_ON_ZERO(const_true(VIRTIO_QWORD(bit) != (qword))) + \
+ BIT_ULL((bit) - 64 * (qword)))
+
+#define VIRTIO_BIT_LO(b) VIRTIO_BIT_QWORD(b, 0)
+#define VIRTIO_BIT_HI(b) VIRTIO_BIT_QWORD(b, 1)
+
#define VIRTIO_DECLARE_FEATURES(name) \
union { \
u64 name; \
--
MST