On Thu, Aug 20, 2015 at 07:21:48AM +0200, Ard Biesheuvel wrote:
On 19 August 2015 at 22:49, Gabriel L. Somlo <somlo@cmu.edu> wrote:
From: "Gabriel L. Somlo" <somlo@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 :)
'random host-supplied blobs' sounds awfully like files in a file system to me, and that is already supported by QEMU and works with any guest OS unmodified. If you are in control of the command line, surely you can add a -drive xxx,fat:path/to/blobs -device xxx pair that simply turns up as a volume.
That did come up, here's the start of original thread on the qemu mailing list from a while back: https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg00371.html To recap, the main advantages to transfering data this way are: 1. Asynchronous The host can simply pass data via the qemu command line, and not have to care if/when the guest is ready to accept the data (i.e. has made it far enough to e.g. start a guest agent) 2. Out-of-band I don't have to take over a user-visible element such as a disk drive. Same reason VSOCK (or VMWare VMCI for that matter) exist and are NOT actual Ethernet/TCP-IP network interfaces :)
DT on ARM is fine, and I'm certainly happy to learn how to do it (even though my main focus is, for now, x86). The unfortunate thing though is that on x86, fw_cfg is *not* AFAICT in ACPI, so I'd have to detour into first adding it in on the host side, before I can rewrite the guest side driver to look it up in there :)
I am not sure how relevant sun4 and ppc/mac are for what you are trying to accomplish, but perhaps it would be best to focus on x86 and ARM for now and do it correctly. If the probing is actually needed, you can always add it later.
I guess that's the direction things seem to be headed, although it would make me a bit sad to leave out sun and ppc right from the very beginning :)
Sorry to be blunt, but I am not convinced there is a need for this driver anyway.
See above (hopefully I'm being sufficiently persuasive :) ) In VMWare one would fetch similar "guestinfo" variables via something like FOO=$(vmware-tools --getinfo "FOO") but I thought exposing fw_cfg in /sys/firmware/... would make access to *any* blobs (including, but not limited to my particular use case) even easier and more generic than that.
PS. If you have one .c file in the kernel which does any of the DT-on-arm boilerplate I'm supposed to immitate, I'd appreciate the shortcut :)
Check out drivers/tty/serial/amba-pl011.c
I'll check it out. Thanks much! --Gabriel