Re: Using Xen's hypercall interface?

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 06/07/2014 23:23, Rig Gel wrote:
> Hi all,
>
> - I'm sorry if this is a repost, I was informed that this is the
> correct mailing list only after sending a msg to xen-users - 
>
> I would like to compile a simple ELF which would make a simple a
> hypercall from a guest in order to
> learn a bit more about the Xen Hypercall interface.
> I tried including the hypervisor.h and -I'ing the ./extras/mini-os/
> dir but it seems that I'm missing a few flags or paths in order ot
> make it fully compile correctly 
>
> Can anyone hint or explain a bit what is the appropriate way to link
> against Xen's header files ? 
> I only need hypervisor.h in order to call some hypercalls 
>
> Attached below a sample of my code -
>
> #include <hypervisor.h>
>
> ...
> snip 
> ...
>
> void test_xen_version(int vers) {
>     HYPERVISOR_xen_version(vers);
> }
>
> ( using the xe_version hypercall is just a mere example, I would like
> to just know how to link/compile correctly against the required header
> files/libs )

It is not possible to make hypercalls from userspace directly.  Allowing
such would cause all kinds of security problems.  Linux and other
operating systems a kernel driver which allow hypercalls via ioctl()s on
/dev/xen/privcmd.

You are best starting with libxenctrl which is a thin userspace library
including OS abstraction to make hypercalls.

>From memory, something like:

#include <stdio.h>
#include <xenctrl.h>

int main(void)
{
    xc_interface *xch = xc_interface_open(NULL, NULL, 0);
    int ver = xc_version(xch, XENVER_version, NULL);

    printf("Xen version %d.%d\n", ver >> 16, ver & 0xffff);
    xc_interface_close(xch);
    return 0;
}

and compiled with `gcc foo.c -o foo -I/path/to/xenctrl.h
-L/path/to/libxenctrl.so -lxenctrl`

ought to get you started.

~Andrew

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