On Tue, Jun 02, 2026 at 08:04:13AM +0200, Eugenio Perez Martin wrote: > On Tue, Jun 2, 2026 at 6:34 AM yangjiale <[email protected]> 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 <[email protected]> > > --- > > 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 )
