adoc

qemu-user-mode.adoc

@@ -0,0 +1,49 @@
+[[qemu-user-mode]]
+= QEMU user mode
+
+This has nothing to do with the Linux kernel, but it is cool:
+
+....
+sudo apt-get install qemu-user
+./build -a arm
+cd buildroot/output.arm~/target
+qemu-arm -L . bin/ls
+....
+
+This uses QEMU's user-mode emulation mode that allows us to run
+cross-compiled userland programs directly on the host.
+
+The reason this is cool, is that `ls` is not statically compiled, but
+since we have the Buildroot image, we are still able to find the shared
+linker and the shared library at the given path.
+
+In other words, much cooler than:
+
+....
+arm-linux-gnueabi-gcc -o hello -static hello.c
+qemu-arm hello
+....
+
+It is also possible to compile QEMU user mode from source with
+`BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE=y`, but then your compilation
+will likely fail with:
+
+....
+package/qemu/qemu.mk:110: *** "Refusing to build qemu-user: target Linux version newer than host's.".  Stop.
+....
+
+since we are using a bleeding edge kernel, which is a sanity check in
+the Buildroot QEMU package.
+
+Anyways, this warns us that the userland emulation will likely not be
+reliable, which is good to know. TODO: where is it documented the host
+kernel must be as new as the target one?
+
+GDB step debugging is also possible with:
+
+....
+qemu-arm -g 1234 -L . bin/ls
+../host/usr/bin/arm-buildroot-linux-uclibcgnueabi-gdb -ex 'target remote localhost:1234'
+....
+
+TODO: find source. Lazy now.