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Subject: [virtio-dev] Re: clarifying the handling of responses for virtio-rpmb


I had my hands in a virtio RPMB device implementation the last few
weeks. During the development process I had to apply some patches to the
virtio RPMB driver:

 * Change the device id from 0xFFFF to 28

 * (Add some debug facilities. Needed to see the frames. Got first no
   request frames on the device side, nothing.)

 * Fix descriptor directions. For the outgoing frames num_in was
   incremented instead of num_out.

The frames in the for-loop may be outgoing or intended for incoming
data. Decided on the RPMB_F_WRITE flag what to do with those frames:

  for (i = 0; i < ncmds; i++) {

        if (cmds[i].flags & RPMB_F_WRITE)
            sgs[num_out++ + num_in] = &frame[i];
            sgs[num_out + num_in++] = &frame[i];

 * Got now too much data comparing to the virtio spec. Removed those
   additional frames in the beginning disabling some pieces of code in
   the virtio RPMB driver.

You are probably puzzled by something which I think is a bug in the
virtio RPMB driver regarding the descriptor directions. Could be that
some device implementations do not really care about provided descriptor
directions, in this case this may go unnoticed for a while.

Am 10.09.20 um 15:08 schrieb Alex BennÃe:
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Alex BennÃe <alex.bennee@linaro.org> writes:


The specification lists a number of commands that have responses:

   The operation of a virtio RPMB device is driven by the requests placed
   on the virtqueue. The type of request can be program key
   (VIRTIO_RPMB_REQ_PROGRAM_KEY), get write counter
   program key or write request can also combine with a result read
   (VIRTIO_RPMB_REQ_RESULT_READ) for a returned result.

Now I'm deep in the guts of virt queues doing the implementation I'm
trying to clarify exactly what frames should be sent to the device and
if they should be out_sgs or in_sgs. I suspect there is some difference
between the original prototype interface and what we have now.

Some operations obviously have an implied response
far as I could tell the frame should be simple:

   sg[0] (out_sg=1) - frame with command
   sg[1..n] (in_sg=1..n) - space for the reply to be filled in by the device

However the language for the program key and data write say they can be
combined with a VIRTIO_RPMB_REQ_RESULT_READ to optionally return a
result. My question is is this result read meant to be in a separate
request frame and response frame so you get:

  sg[1] - VIRTIO_RPMB_REQ_RESULT_READ (out_sg=2)
  sg[2] - empty frame for response (in_sg=1)
This is what works after applying the direction patch above in the
virtio driver and which makes also sense to me. See also below my
comment for the rpmb_ioctl() code.


  sg[1] - VIRTIO_RPMB_REQ_RESULT_READ (in_sg=1)
Makes no sense for me. The VIRTIO_RPMB_REQ_RESULT_READ is a request
(command) in the same way as the other requests.

where the result frame is filled in and sent back?

I must say I'm a little confused by the logic in rpmb_ioctl (in the
userspace tool) which creates both out_frames and resp frames:

Was also confused but it's not that complicated (after some hours). For
added. So in the end as last descriptor there is always an incoming
frame to be filled either with the  RESULT_READ data or the response

   static int rpmb_ioctl(uint8_t frame_type, int fd, uint16_t req,
                         const void *frames_in, unsigned int cnt_in,
                         void *frames_out, unsigned int cnt_out)
           int ret;
           struct __packed {
                   struct rpmb_ioc_seq_cmd h;
                   struct rpmb_ioc_cmd cmd[3];
           } iseq = {};

           void *frame_res = NULL;
           int i;
           uint32_t flags;

           rpmb_dbg("RPMB OP: %s\n", rpmb_op_str(req));
           dbg_dump_frame(frame_type, "In Frame: ", frames_in, cnt_in);

           i = 0;
           flags = RPMB_F_WRITE;
           if (req == RPMB_WRITE_DATA || req == RPMB_PROGRAM_KEY)
                   flags |= RPMB_F_REL_WRITE;
           rpmb_ioc_cmd_set(iseq.cmd[i], flags, frames_in, cnt_in);

           if (req == RPMB_WRITE_DATA || req == RPMB_PROGRAM_KEY) {
                   frame_res = rpmb_frame_alloc(frame_type, 0);
                   if (!frame_res)
                           return -ENOMEM;
                   rpmb_frame_set(frame_type, frame_res,
                                  RPMB_RESULT_READ, 0, 0, 0);
                   rpmb_ioc_cmd_set(iseq.cmd[i], RPMB_F_WRITE, frame_res, 0);

           rpmb_ioc_cmd_set(iseq.cmd[i], 0, frames_out, cnt_out);

           iseq.h.num_of_cmds = i;
           ret = ioctl(fd, RPMB_IOC_SEQ_CMD, &iseq);
           if (ret < 0)
                   rpmb_err("ioctl failure %d: %s.\n", ret, strerror(errno));

           ret = rpmb_check_req_resp(frame_type, req, frames_out);

           dbg_dump_frame(frame_type, "Res Frame: ", frame_res, 1);
           dbg_dump_frame(frame_type, "Out Frame: ", frames_out, cnt_out);
           return ret;

although I'm guessing this might just be an impedance between the
chardev ioctl interface for rpmb and the virtio FE driver which is only
one potential consumer of these rpmb ioc commands?

Can anyone clarify?

Alex BennÃe

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