Re: [Xen-devel] Regarding Xenstore/Xenbus

["Umar Farooq Minhas" <umarfm13@xxxxxxxxxxx>]

Kindly, ignore my previous message. I figured out what was wrong with the 
code. I don't need to call "xs_directory" function because I learned that 
when you access Xenstore from inside domU it has an implicit root at 
"/local/domain/<domid>" so in-order to read 
"/local/domain/<domid>/memory/target" I just need to pass "memory/target" to 
"xs_read" as path.
The thing that I want to know now is that can a userspace tool register and 
be notified of watches on xenstore or is it allowed for kernel level drivers 
only? Apparently, the later seems to be the case because when I try to call 
"xs_watch" from domU it returns with an unknown error. Is there a workaround 
this? If not, I can always setup a loop that polls the path periodically to 
detect changes. I just want to be notified of changes to the path 
"memory/target".
Thanks for your help.

----- Original Message ----- 
From: "Umar Farooq Minhas" <umarfm13@xxxxxxxxxxx>
To: "Vincent Hanquez" <vincent@xxxxxxxxxxxxx>
Cc: <xen-devel@xxxxxxxxxxxxxxxxxxx>
Sent: Wednesday, November 22, 2006 7:56 PM
Subject: Re: [Xen-devel] Regarding Xenstore/Xenbus


> Once again thanks for replying, I appreciate your patience.
>
> Actually I already realized that mistake and replaced "daemon_open" with 
> "domain_open".
> But getting the "unknown error" has now changed into a "segmentation 
> fault" and apparently
> it occurs after i execute the "xs_directory" function to get the path in 
> buf2 variable (line31 below).
> I checked the value of 'len' variable after the call and it turns out to 
> be arbitrarily large.
> What am I doing wrong here ? After replacing "daemon_open" with 
> "domain_open" all the rest
> of the calls to xs.h functions remain the same or are there specific 
> "daemon" and "domain"
> versions of functions? (apparently its the former case)
>
> A few related questions:
>
> -Are there any specific initializations/calls that need to be made from 
> inside domU to first initialize
> and then access xenstore ? Or the functions exposed by xs.h are the only 
> thing required ? Apparently
> xenstore is initialized by dom0 once at startup only.
>
> -Does ability to communicate with xenstore depend on whether the ethernet 
> interface b/w
> dom0 and domU is working or not (since it uses sockets)? Currently my domU 
> can't ping dom0 or any one else
> for that matter.
>
> -I am trying to only read the xenstore(which is allowed to all domains by 
> default), does my problem
> in anyway relate to xenstore access permissions ?
>
> -Are you aware of any user-space management tools available that access 
> xenstore from within domU?
> Sorry for a detailed mail but I am just a beginner and dont know a lot of 
> stuff.
> I'll appreciate your help once again.
>
>
> -------------------------------------------------------------------------------------------
>
> 1   #include <xs.h>
> 2   #include <sys/types.h>
> 3   #include <stdio.h>
> 4   #include <string.h>
> 5
> 6   void main(int argc, char *argv[])
> 7   {
> 8
> 9      struct xs_handle *xs;
> 10     xs_transaction_t th;
> 11     char *path;
> 12     int fd;
> 13     fd_set set;
> 14     int er;
> 15     struct timeval tv = {.tv_sec = 0, .tv_usec = 0 };
> 16     char **vec;
> 17     unsigned int num;
> 18     char * buf;
> 19     char ** buf2;
> 20     unsigned int len;
> 21
> 22     unsigned int domid;
> 23
> 24     printf("Msg1\n");
> 25
> 26     /* Get a connection to the daemon */
> 27     xs = xs_domain_open();
> 28     if ( xs == NULL ) error();
> 29
> 30     th = xs_transaction_start(xs);
> 31         buf2 = xs_directory(xs, th,"/local/domain", &len);
> 32     xs_transaction_end(xs, th, true);
> 33
> 34     int i=0;
> 35
> 36     for(i=0;i<len;i++)
> 37     {
> 38         printf("%s\n",buf2[i]);
> 39     }
> 40
> 41
> 42  // I am running dom0 and oly 1 domU so this condition chooses one of 
> them
> 43     if(len>1)
> 44         domid = atoi(buf2[1]);
> 45     if(len==1)
> 46         domid = atoi(buf2[0]);
> 47         //sprintf( domid, "%ui", buf2[0] );
> 48
> 49     printf("Domid: %i\n", domid);
> 50
> 51     /* Get the local domain path */
> 52     path = xs_get_domain_path(xs, domid);
> 53     if ( path == NULL ) error();
> 54
> 55     printf("Msg3\n");
> 56
> 57     /* Make space for our node on the path */
> 58     path = realloc(path, strlen(path) + strlen("/memory/target") + 1);
> 59     if ( path == NULL ) error();
> 60     strcat(path, "/memory/target");
> 61
> 62     /* Create a watch on /local/domain/%d/mynode. */
> 63     er = xs_watch(xs, path, "mytoken");
> 64     if ( er == 0 ) error();
> 65
> 66     /* We are notified of read availability on the watch via the
> 67      * file descriptor.
> 68      */
> 69     fd = xs_fileno(xs);
> 70
> 71     while (1)
> 72     {
> 73         FD_ZERO(&set);
> 74         FD_SET(fd, &set);
> 75
> 76         /* Poll for data. */
> 77         if ( select(fd + 1, &set, NULL, NULL, &tv) > 0
> 78              && FD_ISSET(fd, &set))
> 79         {
> 80             /* I am not sure how num works -- please describe. */
> 81             vec = xs_read_watch(xs, &num);
> 82             if ( !vec ) error();
> 83             printf("vec contents: %s|%s\n", vec[XS_WATCH_PATH],
> 84                                             vec[XS_WATCH_TOKEN]);
> 85
> 86             /* Prepare a transaction and do a read. */
> 87             th = xs_transaction_start(xs);
> 88                 buf = xs_read(xs, th, vec[XS_WATCH_PATH], &len);
> 89             xs_transaction_end(xs, th, true);
> 90             if ( buf )
> 91             {
> 92                 printf("buflen: %d\nbuf: %s\n", len, buf);
> 93             }
> 94
> 95             /* Prepare a transaction and do a write. */
> 96             /*th = xs_transaction_start(xs);
> 97
> 98  er = xs_write(xs, th, path, "somestuff", strlen("somestuff"));
> 99             xs_transaction_end(xs, th, true);
> 100            if ( er == 0 ) error();*/
> 101        }
> 102    }
> 103
> 104    /* Cleanup */
> 105    close(fd);
> 106    xs_daemon_close(xs);
> 107    free(path);
> 108
> 109 }
> ----- Original Message ----- 
> From: "Vincent Hanquez" <vincent@xxxxxxxxxxxxx>
> To: "Umar Farooq Minhas" <umarfm13@xxxxxxxxxxx>
> Cc: <aball@xxxxxxxxxxxxxxxxxx>; <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Sent: Wednesday, November 22, 2006 10:52 AM
> Subject: Re: [Xen-devel] Regarding Xenstore/Xenbus
>
>
> > On Mon, Nov 20, 2006 at 08:21:20PM -0500, Umar Farooq Minhas wrote:
> > > Thanks for your reply. I was into trying out xenstore acess using
> > > 'libxenstore.so'. I have copied the exact code that I am trying to run
> > > (most of this comes from xenstore wiki). The issue is that I am able to 
> > > run
> > > this code from dom0 but when I try to run it from inside a domU after 
> > > the
> > > first printf output i.e. "Msg1" it terminates abnormally saying "unknown
> > > error". My guess is that its failing when it calls xs_daemon_open(). I
> > > can't figure out what am i missing. Some processes need to be started
> > > before i can acess xenstore? Are there any packages that need to be
> > > installed in domU to access xenstore? I already have 'xen-devel', 
> > > 'xen-lib'
> > > and 'xen-tools' packages installed inside domU. Please help.
> > you need to use "domain_open" instead of "daemon_open" in domU.
> >
> > daemon_open is using a direct unix socket to communicate with xenstored.
> > this socket is only accessible to dom0. domU need to use the other way
> > to communicate with xenstored.
> >
> > Cheers,
> > --
> > Vincent Hanquez

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