Suggestion: Improve hypercall Interface to get real return value

["Yanzhang Li " <liyz@pku.edu.cn>]

Hello everyone,

I am a user of Xen and I have encountered a problem when trying to get the return value from hypercall in TOOLS. I found that when programs in TOOLS called hypercall and failed, it would get return value -1 instead of the real return value given by Hypervisor.

The reason is that the hypercall interface of TOOLS uses ioctl to call hypercall, and ioctl will only return -1 when failure occurs. The related code is in xen-4.2.0/tools/libxc/xc_linux_osdep.c:
    static int linux_privcmd_hypercall(xc_interface *xch, xc_osdep_handle h, privcmd_hypercall_t *hypercall)
    {
        int fd = (int)h;
        return ioctl(fd, IOCTL_PRIVCMD_HYPERCALL, hypercall);
    }

While I don't think this is a good idea, because some tools may wish to use real return value. For example, in xen-4.2.0/tools/memshr/interface.c, the function memshr_vbd_issue_ro_request will switch variable ret which comes from return value of hypercall do_memory_op:
    switch(ret)
    {
        case XENMEM_SHARING_OP_S_HANDLE_INVALID:
            ……
            break;
        case XENMEM_SHARING_OP_C_HANDLE_INVALID:
            ……
            break;
        default:
            break;
}

So I think if we could modify the interface a little bit to return the real error number, it would be beneficial to many TOOLS developers including myself. My suggested modification is simple:
    static int linux_privcmd_hypercall(xc_interface *xch, xc_osdep_handle h, privcmd_hypercall_t *hypercall)
    {
        int fd = (int)h;
        int ret = ioctl(fd, IOCTL_PRIVCMD_HYPERCALL, hypercall);
        if (ret < 0)
            return -errno;
        return ret;
    }

Do you think this would be a good modification? 
Also, I am curious why the original design didn't do that. Is it a bug or is it designed that way intentionally?
Any suggestions and comments will be highly appreciated.

Thanks!
Best Regards,
Yanzhang Li

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