Re: [Qemu-devel] [PATCH v2 2/4] pc-bios/s390-ccw/net: Add support for pxelinux-style config files

[Viktor VM Mihajlovski]

On 24.04.2018 13:23, Thomas Huth wrote:
> On 24.04.2018 13:07, Viktor VM Mihajlovski wrote:
>> On 23.04.2018 09:58, Thomas Huth wrote:
>>> Since it is quite cumbersome to manually create a combined kernel with
>>> initrd image for network booting, we now support loading via pxelinux
>>> configuration files, too. In these files, the kernel, initrd and command
>>> line parameters can be specified seperately, and the firmware then takes
>>> care of glueing everything together in memory after the files have been
>>> downloaded.
>>>
>>> The user can either specify a config file directly as bootfile via DHCP
>>> (but in this case, the file has to start either with "default" or a "#"
>>> comment so we can distinguish it from binary kernels), or a folder (i.e.
>>> the bootfile name must end with "/") where the firmware should look for
>>> the typical pxelinux.cfg file names based on MAC or IP address. If no
>>> direct file or folder has been specified, we still look for certain
>>> files in the default "pxelinux.cfg/" folder, but omit some of the file
>>> names to avoid to download x86 config files here by mistake.
>> I don't think this is necessary, since the DHCP server configuration
>> SHOULD take into consideration the processor architecture. In fact it is
>> even annoying and hard to understand that an attempt is made to load the
>> uuid, mac and "full ip" based config files but not the "abbreviated ip"
>> or default file.
> 
> If the DHCP server has been been properly configured to take the
> processor architecture into account, there should be a usable entry in
> the bootfile (i.e. either a binary, a config file or a folder where
> config files should be probed). So in this case the skipping does not
> take place.
> 
> And if you want the full probing in the pxelinux.cfg/ directory, you can
> also specify "pxelinux.xfg/" in the bootfile entry.
> 
> The skipping only happens if there is no valid entry in the bootfile,
> i.e. the server configuration was likely wrong anyway, and we just do
> some desperate final guesses with the default "pxelinux.cfg/" directory
> before giving up.
> 
My major concern is consistency. The emergency probing is different from
the normal one and this can be hell to debug (it took me a while to
understand what went wrong[1]).
I like the idea to improve the usability by doing a wild guess, but only
if it is consistent with the regular behavior.
>> After all, even if the config file for x86 was loaded,
>> the effect will be that the network boot fails (as it does now).
> 
> Ok, that's true, too.
> 
> Actually, I don't mind too much whether we probe the files based on the
> partial IP address or the "default" name, or whether we skip them. I
> thought it might be a good idea to avoid a potential clash with x86
> files, but if you think that this is rather confusing, I also see your
> point. So I'd say let's wait for some more opinions from others before
> we decide about the final behavior...
> 
>  Thomas
> 

[1] What I did was to keep my previous pxelinux configuration but delete
the pxelinux.0 file on the server to force the built-in processing.

-- 
Regards,
  Viktor Mihajlovski