Re: [Qemu-devel] [PATCH 3/3] pc-bios/s390-ccw/net: Try to load pxelinux.cfg file accoring to the UUID

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 30 May 2018 11:16:58 +0200
Thomas Huth <thuth@redhat.com> wrote:

> With the STSI instruction, we can get the UUID of the current VM instance,
> so we can support loading pxelinux config files via UUID in the file name,
> too.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  pc-bios/s390-ccw/netmain.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/pc-bios/s390-ccw/netmain.c b/pc-bios/s390-ccw/netmain.c
> index e84bb2b..7ece302 100644
> --- a/pc-bios/s390-ccw/netmain.c
> +++ b/pc-bios/s390-ccw/netmain.c
> @@ -235,6 +235,49 @@ static void net_release(filename_ip_t *fn_ip)
>  }
>  
>  /**
> + * Retrieve the Universally Unique Identifier of the VM.
> + * @return UUID string, or NULL in case of errors
> + */
> +static const char *get_uuid(void)
> +{
> +    register int r0 asm("0");
> +    register int r1 asm("1");
> +    uint8_t *mem, *buf, uuid[16];
> +    int i, chk = 0;
> +    static char uuid_str[37];
> +
> +    mem = malloc(2 * PAGE_SIZE);
> +    if (!mem) {
> +        puts("Out of memory ... can not get UUID.");
> +        return NULL;
> +    }
> +    buf = (uint8_t *)(((uint64_t)mem + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1));
> +    memset(buf, 0, PAGE_SIZE);
> +
> +    /* Get SYSIB 3.2.2 */
> +    r0 = (3 << 28) | 2;
> +    r1 = 2;
> +    asm volatile(" stsi 0(%2)\n" : : "d" (r0), "d" (r1), "a" (buf)
> +                 : "cc", "memory");

Maybe check for cc == 3 here, just to be safe? (We can probably assume
a dbct >= 1 if it is successful, I think.)

> +
> +    for (i = 0; i < 16; i++) {
> +        uuid[i] = buf[8 * 4 + 12 * 4 + i];

Is there some way to make this offset less magic (offset of first vmdb
+ offset of uuid inside vmdb)?

> +        chk |= uuid[i];
> +    }
> +    free(mem);
> +    if (!chk) {
> +        return NULL;
> +    }
> +
> +    sprintf(uuid_str, "%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-"
> +            "%02x%02x%02x%02x%02x%02x", uuid[0], uuid[1], uuid[2], uuid[3],
> +            uuid[4], uuid[5], uuid[6], uuid[7], uuid[8], uuid[9], uuid[10],
> +            uuid[11], uuid[12], uuid[13], uuid[14], uuid[15]);
> +
> +    return uuid_str;
> +}
> +
> +/**
>   * Load a kernel with initrd (i.e. with the information that we've got from
>   * a pxelinux.cfg config file)
>   */
> @@ -284,7 +327,8 @@ static int net_try_pxelinux_cfg(filename_ip_t *fn_ip)
>      struct pl_cfg_entry entries[MAX_PXELINUX_ENTRIES];
>      int num_ent, def_ent = 0;
>  
> -    num_ent = pxelinux_load_parse_cfg(fn_ip, mac, NULL, DEFAULT_TFTP_RETRIES,
> +    num_ent = pxelinux_load_parse_cfg(fn_ip, mac, get_uuid(),
> +                                      DEFAULT_TFTP_RETRIES,
>                                        cfgbuf, sizeof(cfgbuf),
>                                        entries, MAX_PXELINUX_ENTRIES, &def_ent);
>      if (num_ent > 0) {