[Virtio-fs] Few queries about virtiofsd read implementation

[Vivek Goyal]

Hi David/Stefan,

I am browsing through the code of read requests (FUSE_READ) in virtiofsd
(and in virtiofs) and I have few questions. You folks probably know the
answers.

1. virtio_send_data_iov(), reads the data from file into the scatter list.
  Some of the code looks strange.

  We seem to be retrying read if we read less number of bytes than what
  client asked for. I am wondering shoudl this really be our
  responsibility or client should deal with it. I am assuming that client
  should be ready to deal with less number of bytes read.

  So what was the thought process behind retrying.

          if (ret < len && ret) {
            fuse_log(FUSE_LOG_DEBUG, "%s: ret < len\n", __func__);
            /* Skip over this much next time around */
            skip_size = ret;
            buf->buf[0].pos += ret;
            len -= ret;

            /* Lets do another read */
            continue;
        }

- After this we have code where if number of bytes read are not same
  as we expect to, then we return EIO.

          if (ret != len) {
            fuse_log(FUSE_LOG_DEBUG, "%s: ret!=len\n", __func__);
            ret = EIO;
            free(in_sg_cpy);
            goto err;
        }

  When do we hit this. IIUC, preadv() will return.

  A. Either number of bytes we expected (no issues)
  B. 0 in case of EOF (We break out of loop and just return to client with
  		      number of bytes we have read so far).
  C. <0 (This is error case and we return error to client)
  D. X bytes which is less than len. 

To handle D we have code to retry. So when do we hit the above if
condition where "ret !=len). Is this a dead code. Or I missed something.

2. When client sent FUSE_READ, we put pointer to pages into sglist. IIUC,
   we put pointer to "struct page *" and not actual page. So who converts
   these struct page pointer to memory belong to page.

   sg_init_fuse_pages() {
   	sg_set_page(&sg[i], pages[i], this_len, page_descs[i].offset);
   }

3. Who converts guest memory address (when) into qemu process address which is
   accessible by virtiofsd.

Thanks
Vivek