Re: [PATCH] docs/system/s390x: Document the "loadparm" machine property

[Janosch Frank]

On 11/14/22 14:25, Thomas Huth wrote:
> The "loadparm" machine property is useful for selecting alternative
> kernels on the disk of the guest, but so far we do not tell the users
> yet how to use it. Add some documentation to fill this gap.
>
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2128235
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>   docs/system/s390x/bootdevices.rst | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
>
> diff --git a/docs/system/s390x/bootdevices.rst 
> b/docs/system/s390x/bootdevices.rst
> index b5950133e8..40089c35a9 100644
> --- a/docs/system/s390x/bootdevices.rst
> +++ b/docs/system/s390x/bootdevices.rst
> @@ -53,6 +53,32 @@ recommended to specify a CD-ROM device via ``-device 
> scsi-cd`` (as mentioned
>   above) instead.
>   
>   
> +Selecting kernels with the ``loadparm`` property
> +------------------------------------------------
> +
> +The ``s390-ccw-virtio`` machine supports the so-called ``loadparm`` parameter
> +which can be used to select the kernel on the disk of the guest that the
> +s390-ccw bios should boot. When starting QEMU, it can be specified like this::
> +
> + qemu-system-s390x -machine s390-ccw-virtio,loadparm=<string>
> +
> +The first way to use this parameter is to use the word ``PROMPT`` as the
> +``<string>`` here. In that case the s390-ccw bios will show a list of
> +installed kernels on the disk of the guest and ask the user to enter a number
> +to chose which kernel should be booted -- similar to what can be achieved by
> +specifying the ``-boot menu=on`` option when starting QEMU. Note that the menu
> +list will only show the names of the installed kernels when using a DASD-like
> +disk image with 4k byte sectors, on normal SCSI-style disks with 512-byte
> +sectors, there is not enough space for the zipl loader on the disk to store
> +the kernel names, so you only get a list without names here.

I'd suggest splitting the last sentence into two so there's a clear 
separation between DASD and SCSI.

> +
> +The second way to use this parameter is to use a number in the range from 0
> +to 31. The numbers that can be used here correspond to the numbers that are
> +shown when using the ``PROMPT`` option, and the s390-ccw bios will then try
> +to automatically boot the kernel that is associated with the given number.
> +Note that ``0`` can be used to boot the default entry.
> +
> +
>   Booting from a network device
>   -----------------------------
>