Re: [Qemu-devel] [PATCH 06/17] migration-local: add send_pipefd()

[Paolo Bonzini]

Il 03/12/2013 12:19, Lei Li ha scritto:
> On 12/02/2013 05:33 PM, Daniel P. Berrange wrote:
>> On Mon, Dec 02, 2013 at 05:19:06PM +0800, Lei Li wrote:
>>> This patch adds send_pipefd() to pass the pipe file descriptor
>>> to destination process.
>>>
>>> Signed-off-by: Lei Li <address@hidden>
>>> ---
>>>   migration-local.c |   46
>>> ++++++++++++++++++++++++++++++++++++++++++++++
>>>   1 files changed, 46 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/migration-local.c b/migration-local.c
>>> index 929ed60..f479530 100644
>>> --- a/migration-local.c
>>> +++ b/migration-local.c
>>> @@ -167,3 +167,49 @@ fail:
>>>       g_free(s);
>>>       return NULL;
>>>   }
>>> +
>>> +
>>> +/*
>>> + * Pass a pipe file descriptor to another process.
>>> + *
>>> + * Return negative value If pipefd < 0. Return 0 on
>>> + * success.
>>> + *
>>> + */
>>> +static int send_pipefd(int sockfd, int pipefd)
>>> +{
>>> +    struct msghdr msg;
>>> +    struct iovec iov[1];
>>> +    ssize_t ret;
>>> +    char req[1] = { 0x01 };
>>> +
>>> +    union {
>>> +      struct cmsghdr cm;
>>> +      char control[CMSG_SPACE(sizeof(int))];
>>> +    } control_un;
>>> +    struct cmsghdr *cmptr;
>>> +
>>> +    msg.msg_control = control_un.control;
>>> +    msg.msg_controllen = sizeof(control_un.control);
>>> +
>>> +    cmptr = CMSG_FIRSTHDR(&msg);
>>> +    cmptr->cmsg_len = CMSG_LEN(sizeof(int));
>>> +    cmptr->cmsg_level = SOL_SOCKET;
>>> +    cmptr->cmsg_type = SCM_RIGHTS;
>>> +    *((int *) CMSG_DATA(cmptr)) = pipefd;
>>> +
>>> +    msg.msg_name = NULL;
>>> +    msg.msg_namelen = 0;
>>> +
>>> +    iov[0].iov_base = req;
>>> +    iov[0].iov_len = sizeof(req);
>>> +    msg.msg_iov = iov;
>>> +    msg.msg_iovlen = 1;
>>> +
>>> +    ret = sendmsg(sockfd, &msg, 0);
>>> +    if (ret <= 0) {
>>> +        DPRINTF("sendmsg error: %s\n", strerror(errno));
>>> +    }
>>> +
>>> +    return ret;
>>> +}
>> Just a reminder about my comments from previous posting. This is
>> introducing a 3rd private function for sending FDs. The existing
>> code should be refactored into qemu-socket.{c,h} and shared.
> 
> Hi Daniel,
> 
> Yes, I remembered your suggestion. As my reply in the previous version,
> I'll make this refactoring in a separate thread. There are some differences
> between these private functions (like data type and length of bytes
> transmitted), may need a little time to get the common method settle down,
> and would be better to do some test to make sure there is no impact on
> them.

You would have to implement it in such a way that the buffer is
specified in the function, for example:

ssize_t qemu_send_with_fd(int sockfd, int passed_fd, const void *buf,
                          size_t len);
ssize_t qemu_recv_with_fd(int sockfd, int *passed_fd, void *buf,
                          size_t len);

The functions can go in util/ (I think not in qemu-socket.c, a new file
is preferrable).

I don't think it's particularly important, but it's definitely welcome.

Paolo