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[Xen-devel] Re: hvmloader/xenbus.c: How to modify or add a key-value pair in xenstore

2011/8/13 Bei Guan <gbtju85@xxxxxxxxx>

In hvmloader/xenbus.c, a simple implementation of xenbus, the function xenbus_send() (see the prototype below) can be used to read a value from xenstore.
static int xenbus_send(uint32_t type, uint32_t len, const char *data, uint32_t *reply_len, const char **reply_data)

Now, I want to modify a key-value existed in xenstore or add a new key-value pair into xenstore, so I write a new function xenbus_write_send(), which is changed from xenbus_send(). It is pasted in the following. The red part is what have changed. However, this function doesn't work well. The "path"=>"value" can be updated or added into xenstore. Is there anything problem with this function? Any reply is appreciated.

struct reqs{
  const char *data;
  unsigned len;

This method can be used to modify the old value of "path" to "value";
And add a new "path"=>"value" pair into xenstore.
static int xenbus_write_send(uint32_t type, const char *path, const char *value,
                       uint32_t *reply_len, const char **reply_data)
    struct xsd_sockmsg hdr;
    evtchn_send_t send;
    int i;
    int p, len, nr_req;

    /* Not acceptable to use xenbus before setting it up */
    ASSERT(rings != NULL);

    struct reqs req[] = {
      {path, strlen(path) + 1},
      {value, strlen(value)},
    len = strlen(path) + 1 + strlen(value);
    nr_req = 2;

    /* Put the request on the ring */
    hdr.type = type;
    hdr.req_id = 0;  /* We only ever issue one request at a time */
    hdr.tx_id = 0;   /* We never use transactions */
    hdr.len = len;
    ring_write((char *) &hdr, sizeof hdr);

    for (p = 0; p < nr_req; p++) {
      ring_write(req[p].data, req[p].len);

    /* Tell the other end about the request */
    send.port = event;
    hypercall_event_channel_op(EVTCHNOP_send, &send);

    /* Properly we should poll the event channel now but that involves
     * mapping the shared-info page and handling the bitmaps. */

    /* Pull the reply off the ring */
    ring_read((char *) &hdr, sizeof(hdr));
    ring_read(payload, hdr.len);
The error message here is EACCES.

    /* For sanity's sake, nul-terminate the answer */
    payload[hdr.len] = '\0';

    /* Handle errors */
    if ( hdr.type == XS_ERROR )
        *reply_len = 0;
        for ( i = 0; i < ((sizeof xsd_errors) / (sizeof xsd_errors[0])); i++ )
            if ( !strcmp(xsd_errors[i].errstring, payload) )
                return xsd_errors[i].errnum;
        /* Default error value if we couldn't decode the ASCII error */
        return EIO;

    *reply_data = NULL;
    *reply_len = hdr.len;
    return 0;

Best Regards,
Bei Guan

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