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: > > On Wed, Aug 19, 2015 at 11:42:02AM +0200, Ard Biesheuvel wrote: > >> On 19 August 2015 at 11:38, Ard Biesheuvel <ard.biesheu...@linaro.org> > >> wrote: > >>> From: "Gabriel L. Somlo" <so...@cmu.edu> > >>>> 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
