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Re: [Qemu-devel] [RFC PATCH 0/2] host->guest environment variables via f

From: Gabriel L. Somlo
Subject: Re: [Qemu-devel] [RFC PATCH 0/2] host->guest environment variables via fw_cfg
Date: Wed, 25 Feb 2015 20:13:20 -0500
User-agent: Mutt/1.5.23 (2014-03-12)

On Wed, Feb 25, 2015 at 10:40:34PM +0000, Matt Fleming wrote:
> Right, I was thinking about this in the context of configuring the BIOS
> (OVMF in my case) with runtime tunable knobs instead of having to
> recompile the BIOS image from scratch.
> I'll carve out some time to review your patches Gabriel.

Thanks ! Although the more I think about it, the more I like the idea
of simply adding a generic named blob to fw_cfg from the host side
(rather than adding a dedicated "-guestenv 'foo=bar'" option.

If I make sure the generic "-fwcfg name='blob-name',file=./blob_file"
is processed *after* everything else is already inserted into fw_cfg,
and that we throw an error if 'blob-name' collides with anything
already added during qemu setup, there's no reason I can't pass a
blob named 'etc/guest-info'.

So I'll send out a v2 attempting to do that, which should take care of
both of our needs on the host side.

On the guest side, you should be taken care of -- read whatever you
need to read from fw_cfg using the BIOS's existing mechanisms.

For myself, I'm tempted to write a kernel driver to allow me to simply
"cat /sys/firmware/fw_cfg/etc/guestinfo | grep '^key_name='" when I
need to retrieve a guest environment variable.

However, I'd first need to figure out how to do the equivalent thing
on Windows, and whether the requirement to add a kernel module is
worth it when my main purpose is making it easy to recycle VMWare VMs
which use "vmware-tools --cmd info-get guestinfo.key_name" :)

Maybe it's better to stick with accessing the fw_cfg io ports from
guest-side userspace, so patch 2/2 should stand as is (modulo Daniel's
suggestion to make it a separate binary from "qemu-ga", of course).


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