On Wed, Jan 28, 2026 at 02:59:46PM +0100, Johannes Thumshirn wrote: > From: Alexander Graf <[email protected]> > > KCSAN reports data races when accessing virtio ring fields that are > concurrently written by the device (host). These are legitimate > concurrent accesses where the CPU reads fields that the device updates > via DMA-like mechanisms. > > Add accessor functions that use READ_ONCE() to properly annotate these > device-writable fields and prevent compiler optimizations that could
let's add "in theory" here > break the code. This also serves as documentation showing which fields > are shared with the device. > > The affected fields are: > - Split ring: used->idx, used->ring[].id, used->ring[].len > - Packed ring: desc[].flags, desc[].id, desc[].len I would add here: using WRITE_ONCE for driver writes into fields isn't done here as it does not currently trigger warnings. > > Reported-by: Kernel Concurrency Sanitizer (KCSAN) > Signed-off-by: Alexander Graf <[email protected]> > [jth: Add READ_ONCE in virtqueue_kick_prepare_split ] > Signed-off-by: Johannes Thumshirn <[email protected]> Looks good to me. Yet something to improve: > --- > drivers/virtio/virtio_ring.c | 88 ++++++++++++++++++++++++++++++------ > 1 file changed, 73 insertions(+), 15 deletions(-) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index ddab68959671..74957c83e138 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -222,6 +222,63 @@ struct vring_virtqueue { > #endif > }; > > +/* > + * Accessors for device-writable fields in virtio rings. > + * These fields are concurrently written by the device and read by the > driver. > + * Use READ_ONCE() to prevent compiler optimizations and document the > + * intentional data race. ... and prevent KCSAN warnings > + */ > + > +/* Split ring: read device-written fields from used ring */ > +static inline u16 vring_used_idx_read(const struct vring_virtqueue *vq) I'd prefer a consistent prefix. Maybe: vring_read_used_idx and so on. > +{ > + return virtio16_to_cpu(vq->vq.vdev, > + READ_ONCE(vq->split.vring.used->idx)); > +} > + > +static inline u32 vring_used_id_read(const struct vring_virtqueue *vq, > + u16 idx) > +{ > + return virtio32_to_cpu(vq->vq.vdev, > + READ_ONCE(vq->split.vring.used->ring[idx].id)); > +} > + > +static inline u32 vring_used_len_read(const struct vring_virtqueue *vq, > + u16 idx) > +{ > + return virtio32_to_cpu(vq->vq.vdev, > + READ_ONCE(vq->split.vring.used->ring[idx].len)); > +} > + > +/* Packed ring: read device-written fields from descriptors */ > +static inline u16 vring_packed_desc_flags_read(const struct vring_virtqueue > *vq, > + u16 idx) > +{ > + return le16_to_cpu(READ_ONCE(vq->packed.vring.desc[idx].flags)); > +} > + > +static inline u16 vring_packed_desc_id_read(const struct vring_virtqueue *vq, > + u16 idx) > +{ > + return le16_to_cpu(READ_ONCE(vq->packed.vring.desc[idx].id)); > +} > + > +static inline u32 vring_packed_desc_len_read(const struct vring_virtqueue > *vq, > + u16 idx) > +{ > + return le32_to_cpu(READ_ONCE(vq->packed.vring.desc[idx].len)); > +} > + > +/* > + * Note: We don't need READ_ONCE for driver->device fields like: > + * - split.vring.avail->idx (driver writes, device reads) > + * - packed.vring.desc[].addr (driver writes, device reads) > + * These are written by the driver and only read by the device, so the > + * driver can safely access them read them, really. > without READ_ONCE. ... and without triggering KCSAM warnings. > The device must use > + * appropriate barriers on its side. > + */ > + > + extra empty line here. > static struct vring_desc_extra *vring_alloc_desc_extra(unsigned int num); > static void vring_free(struct virtqueue *_vq); > > @@ -736,9 +793,10 @@ static bool virtqueue_kick_prepare_split(struct > virtqueue *_vq) > LAST_ADD_TIME_INVALID(vq); > > if (vq->event) { > - needs_kick = vring_need_event(virtio16_to_cpu(_vq->vdev, > - vring_avail_event(&vq->split.vring)), > - new, old); > + u16 event = virtio16_to_cpu(_vq->vdev, > + READ_ONCE(vring_avail_event(&vq->split.vring))); why not wrap this one, too? > + > + needs_kick = vring_need_event(event, new, old); > } else { > needs_kick = !(vq->split.vring.used->flags & > cpu_to_virtio16(_vq->vdev, > @@ -808,8 +866,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, > unsigned int head, > > static bool more_used_split(const struct vring_virtqueue *vq) > { > - return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev, > - vq->split.vring.used->idx); > + return vq->last_used_idx != vring_used_idx_read(vq); > } > > static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, > @@ -838,10 +895,8 @@ static void *virtqueue_get_buf_ctx_split(struct > virtqueue *_vq, > virtio_rmb(vq->weak_barriers); > > last_used = (vq->last_used_idx & (vq->split.vring.num - 1)); > - i = virtio32_to_cpu(_vq->vdev, > - vq->split.vring.used->ring[last_used].id); > - *len = virtio32_to_cpu(_vq->vdev, > - vq->split.vring.used->ring[last_used].len); > + i = vring_used_id_read(vq, last_used); > + *len = vring_used_len_read(vq, last_used); > > if (unlikely(i >= vq->split.vring.num)) { > BAD_RING(vq, "id %u out of range\n", i); > @@ -923,8 +978,7 @@ static bool virtqueue_poll_split(struct virtqueue *_vq, > unsigned int last_used_i > { > struct vring_virtqueue *vq = to_vvq(_vq); > > - return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev, > - vq->split.vring.used->idx); > + return (u16)last_used_idx != vring_used_idx_read(vq); > } > > static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq) > @@ -1701,10 +1755,10 @@ static void detach_buf_packed(struct vring_virtqueue > *vq, > static inline bool is_used_desc_packed(const struct vring_virtqueue *vq, > u16 idx, bool used_wrap_counter) > { > - bool avail, used; > u16 flags; > + bool avail, used; > > - flags = le16_to_cpu(vq->packed.vring.desc[idx].flags); > + flags = vring_packed_desc_flags_read(vq, idx); > avail = !!(flags & (1 << VRING_PACKED_DESC_F_AVAIL)); > used = !!(flags & (1 << VRING_PACKED_DESC_F_USED)); > > @@ -1751,8 +1805,8 @@ static void *virtqueue_get_buf_ctx_packed(struct > virtqueue *_vq, > last_used_idx = READ_ONCE(vq->last_used_idx); > used_wrap_counter = packed_used_wrap_counter(last_used_idx); > last_used = packed_last_used(last_used_idx); > - id = le16_to_cpu(vq->packed.vring.desc[last_used].id); > - *len = le32_to_cpu(vq->packed.vring.desc[last_used].len); > + id = vring_packed_desc_id_read(vq, last_used); > + *len = vring_packed_desc_len_read(vq, last_used); > > if (unlikely(id >= vq->packed.vring.num)) { > BAD_RING(vq, "id %u out of range\n", id); > @@ -1850,6 +1904,10 @@ static bool virtqueue_poll_packed(struct virtqueue > *_vq, u16 off_wrap) > bool wrap_counter; > u16 used_idx; > > + /* > + * Note: off_wrap is from virtqueue_enable_cb_prepare_packed() which > + * already used READ_ONCE on vq->last_used_idx, so we don't need it > again. we don't need what again? off_wrap is a local variable. > + */ > wrap_counter = off_wrap >> VRING_PACKED_EVENT_F_WRAP_CTR; > used_idx = off_wrap & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR); > > -- > 2.52.0
