From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53377) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZUfE8-0000Sn-Gv for qemu-devel@nongnu.org; Wed, 26 Aug 2015 14:15:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZUfE5-0008Oj-Ae for qemu-devel@nongnu.org; Wed, 26 Aug 2015 14:15:12 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:41001) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZUfE5-0008OY-2E for qemu-devel@nongnu.org; Wed, 26 Aug 2015 14:15:09 -0400 Message-ID: <55DE0227.8090905@codeaurora.org> Date: Wed, 26 Aug 2015 14:15:03 -0400 From: Christopher Covington MIME-Version: 1.0 References: <1439977109-20314-1-git-send-email-ard.biesheuvel@linaro.org> <20150819204915.GA6164@GLSMBP.INI.CMU.EDU> In-Reply-To: <20150819204915.GA6164@GLSMBP.INI.CMU.EDU> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 0/3] SysFS driver for QEMU fw_cfg device List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Gabriel L. Somlo" , Ard Biesheuvel Cc: paul@pwsan.com, Matt Fleming , "Richard W.M. Jones" , Leif Lindholm , Jordan Justen , "x86@kernel.org" , QEMU Developers , gleb@cloudius-systems.com, galak@codeaurora.org, kernelnewbies@kernelnewbies.org, Gerd Hoffmann , "gregkh@linuxfoundation.org" , Paolo Bonzini , zajec5@gmail.com, linux-api@vger.kernel.org, Laszlo Ersek , "linux-kernel@vger.kernel.org" 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 wrote: >>> From: "Gabriel L. Somlo" >>>> 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