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Re: [Qemu-devel] [PATCH v2] vl.c: disallow command line fw cfg without o

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v2] vl.c: disallow command line fw cfg without opt/
Date: Thu, 17 Mar 2016 10:49:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 03/17/16 09:42, Gerd Hoffmann wrote:
>   Hi,
>>> Having a mis-use of this feature become "institutionalized" over time was
>>> seen as a low/negligible risk at the time. Do we have any new reasons
>>> to worry about it ?
>> OVMF uses this feature for a few flags. They are all called
>> "opt/ovmf/...". I followed the advice in "docs/specs/fw_cfg.txt" (which
>> shouldn't be surprising since I seem to have reviewed every patch for
>> that file):
> seabios has a few flags too, in etc/.  Some of them are supported
> directly by qemu (such as setting the boot menu delay).

I forgot to confirm: clearly, when some flag already exists on which we
can foist, kicking and screaming, a somewhat similar interpretation in
OVMF, we do that without inventing a new flag. The boot menu delay is
like this. Namely, if you set "-boot menu=on,splash-time=5000", you will
get a progress bar for five seconds before auto-booting the otherwise
relevant boot option.

The QEMU manual documents the -boot option as guest firmware-specific,
and OVMF operates with that in mind -- support what it can, as closely
it can.


> Others are not,
> and I'd like to be able to use -fw_cfg for them for testing/debugging
> (any use cases beyond that should be supported by adding a less obscure
> way to set them to qemu, similar to the boot delay).
> Oh, and that'll most likely be more seabios testing than qemu testing,
> so why require me build a special qemu version for that?
> On the ovmf flags:  What kind of flags are there?  Anything a normal
> user might want to set?
> cheers,
>   Gerd

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