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Tsirkin wrote: > On Tue, Jun 02, 2026 at 08:04:13AM +0200, Eugenio Perez Martin wrote: >> On Tue, Jun 2, 2026 at 6:34 AM yangjiale wrote: >>> >>> When a descriptor list spans across cache lines, >>> updating the flag first can lead to a scenario where the device side >>> perceives the flag as valid, yet the corresponding address and length >>> fields remain unupdated—resulting in invalid values. >>> Therefore, the flag field must be updated last. >>> >>> Signed-off-by: yangjiale >>> --- >>> drivers/virtio/virtio_ring.c | 8 ++++---- >>> 1 file changed, 4 insertions(+), 4 deletions(-) >>> >>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >>> index fbca7ce1c6bf..036b4f90d30f 100644 >>> --- a/drivers/virtio/virtio_ring.c >>> +++ b/drivers/virtio/virtio_ring.c >>> @@ -1688,6 +1688,10 @@ static inline int virtqueue_add_packed(struct vring_virtqueue *vq, >>> &addr, &len, premapped, attr)) >>> goto unmap_release; >>> >>> + desc[i].addr = cpu_to_le64(addr); >>> + desc[i].len = cpu_to_le32(len); >>> + desc[i].id = cpu_to_le16(id); >>> + >>> flags = cpu_to_le16(vq->packed.avail_used_flags | >>> (++c == total_sg ? 0 : VRING_DESC_F_NEXT) | >>> (n < out_sgs ? 0 : VRING_DESC_F_WRITE)); >>> @@ -1696,10 +1700,6 @@ static inline int virtqueue_add_packed(struct vring_virtqueue *vq, >>> else >>> desc[i].flags = flags; >>> >>> - desc[i].addr = cpu_to_le64(addr); >>> - desc[i].len = cpu_to_le32(len); >>> - desc[i].id = cpu_to_le16(id); >>> - >>> if (unlikely(vq->use_map_api)) { >>> vq->packed.desc_extra[curr].addr = premapped ? >>> DMA_MAPPING_ERROR : addr; >> >> These flags are updated before the flags of the head descriptor at the >> end of the function, at "vq->packed.vring.desc[head].flags = >> head_flags", so the device should not see these. Because of that, the >> relative order between the rest of the fields of the same descriptor >> or other descriptors' fields, except for the head descriptor's flags, >> should not matter. There is a write memory barrier just before >> updating the head's flags. >> >> Also, I don't get why the cache line matters here. Can you expand? Am >> I missing something? > > Oh desc is just a local thing. ENOCOFFEE ) > > Whether or not this can happen in practice, I notice virtqueue_add_packed_in_order() has the same write pattern. Dongli Zhang