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UaU1O0NTl0QUlhtg8GLjcGq8FMLgMygwArRIhMVU= On 6/1/2026 11:04 PM, 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. The above analysis is absolutely correct. Though one hardware vendor told me that this driver implementation kinda stops them from reading ahead of descriptors already posted beyond the available index., ending up with suboptimal performance that is hard to make up by other means. Would it be a bad idea to go with this change and add write barrier in a gentle way for a small flit in the batch, e.g. commit to memory after every cache line size worth of descriptors are posted? Would the memory barrier have negative performance overhead to other backend implementation variants than real hardware PCI device? -Siwei > > Also, I don't get why the cache line matters here. Can you expand? Am > I missing something?