Re: [Xen-devel] [help] problem with `tools/xenstore/xs.c: xs_talkv()`

[éç <aware.why@xxxxxxxxx>]

On Thu, Aug 9, 2012 at 4:21 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

On Thu, 2012-08-09 at 08:07 +0100, 马磊 wrote:
>
>
> On Wed, Aug 8, 2012 at 4:32 PM, 马磊 <aware.why@xxxxxxxxx> wrote:
>         Hi all,
>
>             In  xen-4.1.2 src/tools/xenstore/xs.c: xs_talkv function,
>          there are several lines as follow:
>
>
>          437        mutex_lock(&h->request_mutex);
>
>
>
>          438
>
>
>
>          439        if (!xs_write_all(h->fd, &msg, sizeof(msg)))
>
>
>
>          440                goto fail;
>
>
>
>          441
>
>
>
>          442        for (i = 0; i < num_vecs; i++)
>
>
>
>          443                if (!xs_write_all(h->fd,
>         iovec[i].iov_base, iovec[i].iov_len))
>
>
>
>          444                        goto fail;
>
>
>
>          445
>
>
>
>          446        ret = read_reply(h, &msg.type, len);
>
>
>
>          447        if (!ret)
>
>
>
>          448                goto fail;
>
>
>
>          449
>
>
>
>          450        mutex_unlock(&h->request_mutex);
>
>
>
>
>         The above seems to tell me that after writing to h->fd , the
>         read_reply invoking read_message which immediatelly  read from
>         hd->fd?
>         What did it mean by this?
>
>
>
>
>         Thanks
>
> If hd->fd refers to a socket descriptor, it's understandable that
> writing and then imediatelly reading, in which case the fd is returned
> by get_handle(xs_daemon_socket(), flags).
>
>
> But when fd is retrived by get_handle(xs_domain_dev(), flags), it
> means to write to a file and then read from the same file imediatelly.
> Dose it have something to do with the internal communication
> protocol?!

Yes, the xenstore protocol involves both writing messages and reading
replies, but that seems trivially obvious so I'm afraid I really have no
idea what your question is nor what is confusing you. Perhaps describing
in more detail what you are trying to achieve will help?

Reading http://wiki.xen.org/wiki/Asking_Xen_Devel_Questions might also
help you consider what it is you are asking.


>
>
> Thanks for replying.
>
>
>
>
>
>

Sorry about that, pehaps it results from my lacking of rudimentary knowledge.

IIRC, when using socket, there are two buffers of which one is for sending and another for receiving, so I  said 'If hd->fd refers to a socket descriptor, it's understandable that

> writing and then imediatelly reading,'

But  when fd is retrived by get_handle(xs_domain_dev(), flags), it seems that sender and receiver both writing to the same file, I have no idea about the file /proc/xen/xenbus for communicating between domains?

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