On Wed, Jan 28, 2026 at 03:47:27PM +0100, Alexander Graf wrote: > > On 28.01.26 14:59, 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 > > 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 > > > > Reported-by: Kernel Concurrency Sanitizer (KCSAN) > > Signed-off-by: Alexander Graf <[email protected]> > > > Thanks for persistently trying to fix these KCSAN warnings! :) > > This patch was an initial AI generated stab at seeing whether READ_ONCE > would work and how to make it pretty. It was not meant to go to the mailing > list as is. Some comments on what we would need to improve to bring it to a > mergeable state.
According to latest Docs, use of AI should be documented. Just a statement to this end is probably going to be enough. > > Given this is not a subsystem-contributor relationship, I also think it > would be Co-developed-by instead of signed-off-by :). > > > [jth: Add READ_ONCE in virtqueue_kick_prepare_split ] > > Signed-off-by: Johannes Thumshirn <[email protected]> > > --- > > 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. > > > Should mention that this is necessary for KCSAN. > > > > + */ > > + > > +/* Split ring: read device-written fields from used ring */ > > > Useless comment > > > > +static inline u16 vring_used_idx_read(const struct vring_virtqueue *vq) > > > Just do a complete sed s/_read// on this patch. Nobody needs these _read > suffixes. That's fine, too. > > > +{ > > + 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 */ > > > Useless comment > > > > +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 without READ_ONCE. The device must use > > + * appropriate barriers on its side. > > + */ > > > Useless comment really. If you think it's worthwhile to mention the above, > put it into the patch description. > > > > + > > + > > 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))); > > + > > + 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. > > + */ > > > On its own in this code base 5 years from now, this comment will be super > confusing. Because nobody has context what this note is about. I'd say just > remove it. > > > Alex > > > > wrap_counter = off_wrap >> VRING_PACKED_EVENT_F_WRAP_CTR; > > used_idx = off_wrap & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR); > > > > -- > > 2.52.0 > > > > > > Amazon Web Services Development Center Germany GmbH > Tamara-Danz-Str. 13 > 10243 Berlin > Geschaeftsfuehrung: Christof Hellmis, Andreas Stieger > Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B > Sitz: Berlin > Ust-ID: DE 365 538 597
