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:

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

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On 19 August 2015 at 11:38, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

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From: "Gabriel L. Somlo" <somlo@cmu.edu>

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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