On Tue, Sep 24, 2024 at 9:32 AM Xuan Zhuo [off-list ref] wrote:

The current configuration sets the virtqueue (vq) to premapped mode,
implying that all buffers submitted to this queue must be mapped ahead
of time. This presents a challenge for the virtnet send queue (sq): the
virtnet driver would be required to keep track of dma information for vq
size * 17, which can be substantial. However, if the premapped mode were
applied on a per-buffer basis, the complexity would be greatly reduced.
With AF_XDP enabled, AF_XDP buffers would become premapped, while kernel
skb buffers could remain unmapped.

We can distinguish them by sg_page(sg), When sg_page(sg) is NULL, this
indicates that the driver has performed DMA mapping in advance, allowing
the Virtio core to directly utilize sg_dma_address(sg) without
conducting any internal DMA mapping. Additionally, DMA unmap operations
for this buffer will be bypassed.

So I think we still need some explanation here. I think this works for
virtio-net as the sgs are initialized by the virtio-net device itself.

But it seems not the case for all the others where the sgs were passed
from the uppyer subsystem. For example in __virtscsi_add_cmd(), we
had:

        if (sc && sc->sc_data_direction != DMA_NONE) {
        if (sc->sc_data_direction != DMA_FROM_DEVICE)
                out = &sc->sdb.table;
        if (sc->sc_data_direction != DMA_TO_DEVICE)
                in = &sc->sdb.table;
        }

        /* Request header.  */
        sg_init_one(&req, &cmd->req, req_size);
        sgs[out_num++] = &req;

        /* Data-out buffer.  */
        if (out) {
                /* Place WRITE protection SGLs before Data OUT payload */
                if (scsi_prot_sg_count(sc))
                        sgs[out_num++] = scsi_prot_sglist(sc);
                sgs[out_num++] = out->sgl;
        }

Thanks