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Message-ID: <20260128121309-mutt-send-email-mst@kernel.org>
Date: Wed, 28 Jan 2026 12:23:35 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Johannes Thumshirn <johannes.thumshirn@....com>
Cc: Jason Wang <jasowang@...hat.com>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Eugenio Pérez <eperezma@...hat.com>,
"open list:VIRTIO CORE" <virtualization@...ts.linux.dev>,
open list <linux-kernel@...r.kernel.org>,
Alexander Graf <graf@...zon.com>
Subject: Re: [PATCH] virtio_ring: Add READ_ONCE annotations for
device-writable fields
On Wed, Jan 28, 2026 at 02:59:46PM +0100, Johannes Thumshirn wrote:
> From: Alexander Graf <graf@...zon.com>
>
> 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 <graf@...zon.com>
> [jth: Add READ_ONCE in virtqueue_kick_prepare_split ]
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
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
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