Re: [PATCH v2 0/3] SysFS driver for QEMU fw_cfg device

Hi Gabriel,

On 08/19/2015 04:49 PM, Gabriel L. Somlo wrote:

Hi Ard,

On Wed, Aug 19, 2015 at 11:42:02AM +0200, Ard Biesheuvel wrote:

quoted

(missed some cc's)

On 19 August 2015 at 11:38, Ard Biesheuvel [off-list ref] wrote:

quoted

From: "Gabriel L. Somlo" <redacted>

quoted

Several different architectures supported by QEMU are set up with a
"firmware configuration" (fw_cfg) device, used to pass configuration
"blobs" into the guest by the host running QEMU.

Historically, these config blobs were mostly of interest to the guest
BIOS, but since QEMU v2.4 it is possible to insert arbitrary blobs via
the command line, which makes them potentially interesting to userspace
(e.g. for passing early boot environment variables, etc.).

Does 'potentially interesting' mean you have a use case? Could you elaborate?

My personal one would be something like:

cat > guestinfo.txt << EOT
  KEY1="val1"
  KEY2="val2"
  ...
EOT

qemu-system-x86_64 ... -fw-cfg name="opt/guestinfo",file=./guestinfo.txt ...

Then, from inside the guest:

  . /sys/firmware/qemu_fw_cfg/by_name/opt/guestinfo/raw

  do_something_with $KEY1 $KEY2
  ...

But I'm thinking this is only one of the many positive things one
could do with the ability to access random host-supplied blobs from
guest userspace :)

I do this with kernel parameters:

host:
qemu-system-aarch64 -append="KEY1=val1 KEY2=val2"

guest:
KEY1=`sed -nr s/.*KEY1=([^ ]+).*/\1/ /proc/cmdline`
KEY2=`sed -nr s/.*KEY2=([^ ]+).*/\1/ /proc/cmdline`

do_something_with $KEY1 $KEY2

In practice it's just script=hostfile, where hostfile is available to the
guest via a 9P passthrough filesystem mount.

While quite architecture specific, I've also previously used an
"angel-cmdline" tool for similar purposes. Peter's recent semihosting patches
support such a tool for AArch64. (On AArch32 upstream QEMU disallows
semihosting from userspace.)

Before I had 9P on all the simulators I regularly ran, I used a semihosting
based "angel-load" tool.

Regards,
Christopher Covington

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