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: >> (missed some cc's) >> >> On 19 August 2015 at 11:38, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: >>> From: "Gabriel L. Somlo" <somlo@xxxxxxx> >>>> 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 -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
