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Re: [Qemu-devel] RFC: Proposal to add QEMU "Guest Environment Variables"

From: Christopher Covington
Subject: Re: [Qemu-devel] RFC: Proposal to add QEMU "Guest Environment Variables"
Date: Wed, 04 Feb 2015 10:55:41 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:33.0) Gecko/20100101 Thunderbird/33.0

Hi Gabriel,

Summary up top, why I like the approach below.

On 02/03/2015 02:09 PM, Gabriel L. Somlo wrote:
> Hi,

> For this functionality to really resemble the way environment variables
> are used, environment information should be provided as part of the QEMU
> command line, without the requirement to make a separate/subsequent
> connection to the guest agent. For example:
>     qemu-system-x86_64 -guest-env="VAR1=value1;VAR2=value 2" -hda image.qcow2

qemu-system-x86_64 -append "myvars=VAR1=value1;VAR2=value2" -hda image.qcow2

> Once the guest is started, it should be possible to query the guest
> environment with something like:
>     $ qemu-guest-env get VAR1
>     value1
>     $ qemu-guest-env get VAR2
>     value 2

eval `sed -nr 's/myvars=(.*)/\1/' /proc/cmdline`
echo $VAR1
echo $VAR2

I have two use cases that I've enabled or would like to enable across a
variety of platforms and architectures:

A) Script workload(s) that should run from a regular "cold" boot.
B) On slow systems, script the workload(s) that should be run immediately
following a checkpoint restore ("warm" boot).

Originally I was using the ARM Angel semihosting command line for both A and
B. But that required a JTAG debugger be attached to hardware devices, so I
switched to appending foo=bar strings to /proc/cmdline, which has the
additional benefit of being portable across architectures. Does QEMU's -append
flag for setting and something like `sed -nr 's/myopts=(.*)/\1/'
/proc/cmdline` for getting work for your use case?

I've wondered about something portable across architectures like QMP or
VirtIO-something for the B use case, but that hasn't really progressed beyond
thought experiment.


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