Hi Christopher,

On Wed, Aug 26, 2015 at 02:15:03PM -0400, Christopher Covington wrote:

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

quoted

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

quoted

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

quoted

From: "Gabriel L. Somlo" <somlo-D+Gtc/HYRWM@public.gmane.org>

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.

Someone (maybe it was you) did suggest this during an early thread on
the QEMU dev list. I had considered this, then thought about
piggybacking on smbios (e.g. the type 40 "additional information"
table), but then realized "wait, smbios is currently being inserted
into the guest via fw_cfg, so maybe direct fw_cfg blob transfer *is*
the most asynchronous and out-of-band way I can do this... :)

True, writing a fw_cfg driver is still Linux-specific (leaves out
Windows, which is something I still care about), but dumping a fw_cfg
blob using 'cat' or 'cp' on linux has a certain appeal :)

Yeah, the immediate use case I have personally can be worked around
with kernel command line args, in a slightly less out-of-band, but
still quite serviceable way. I'm thinking a fw_cfg driver is just a
flexible way to make other use cases possible in a (IMHO) neat and
clean way...

Plus, it got me to learn about kobjects, now studying DTs on ARM, so
it's fun ;)

Thanks,
--Gabriel