Further improve gem5 documentation

gem5.md

@@ -15,11 +15,14 @@ Vs QEMU:
 
     It is used mostly for research purposes: when you are making a new chip technology, you don't really need to specialize enormously to an existing microarchitecture, but rather develop something that will work with a wide range of future architectures.
 
--   disadvantage: slower than QEMU by TODO 10x? Which implies:
+-   disadvantage: slower than QEMU by TODO 10x?
 
-    - GEM5 is used only by chip makers, who keep everything that really works closed, and researchers, who can't version track or document code properly >:-). So the documentation is much more scarce.
+    This also implies that the user base is much smaller, since no Android devs.
 
-    - the user base is much smaller (no Android devs!), and so it takes longer to support new hardware features, and reach newer kernel compatibility.
+    Instead, we have only chip makers, who keep everything that really works closed, and researchers, who can't version track or document code properly >:-) And this implies that:
+
+    - the documentation is more scarce
+    - it takes longer to support new hardware features
 
 ## ARM
 
@@ -30,22 +33,6 @@ On another shell:
 
     ./rungem5-shell
 
-This is the best guide so far: <http://www.gem5.org/ARM_Kernel>
-
-    git checkout gem5-v4.9
-
-Linux:
-
-    cd linux
-    git checkout 69973b830859bc6529a7a0468ba0d80ee5117826
-
-Version found by: go down on branch gem5/v4.9 of <https://gem5.googlesource.com/arm/linux> until you find Linus :-) The patches there are just simple optimizations and instrumentation, but they are not needed to boot.
-
-    cd buildroot
-    git checkout 73b075737e23814a68c66e481230af662e1529cb
-
-Version found by: search for the message of type `"linux: bump default to version 4.9.6"`. This changes `BR2_LINUX_KERNEL_LATEST_VERSION` in `/linux/Config.in`.
-
 ### Kernel command line arguments
 
 TODO no matter what argument I pass to: `--command-line`, e.g.  even an innocent `--command-line='printk.time=y'`, it fails with:
@@ -66,15 +53,19 @@ TODO no matter what argument I pass to: `--command-line`, e.g.  even an innocent
 
 ### QEMU with GEM5 kernel configuration
 
-TODO:
+TODO: QEMU did not work with the GEM5 kernel configurations.
+
+To test this, hack up `run` to use the `buildroot/output.arm-gem5~` directory, and then run:
 
     ./run -a arm
 
-hangs at:
+Now QEMU hangs at:
 
     audio: Could not init `oss' audio driver
 
-and the display shows "  "
+and the display shows:
+
+    Guest has not initialized the display (yet).
 
 ### GEM5 with QEMU kernel configuration
 

kernel_config_gem5

@@ -1,4 +1,6 @@
-# Copied from: 
+# Copied from: https://gem5.googlesource.com/arm/linux/+/917e007a4150d26a0aa95e4f5353ba72753669c7/arch/arm/configs/gem5_defconfig
+# which is on branch v4.9, which is a few commits ahead of Linus' v4.9.
+# The patches there are just simple optimizations and instrumentation, but they are not needed to boot.
 
 #
 # Automatically generated file; DO NOT EDIT.

maintainers.md

@@ -25,6 +25,22 @@ This backwards compatibility is just awesome, it makes getting and running the l
 
 This also makes this repo the perfect setup to develop the Linux kernel.
 
+## How to downgrade the Linux kernel?
+
+The kernel is not forward compatible, however, so downgrading the Linux kernel requires downgrading the userland too to the latest Buildroot branch that supports it.
+
+The default Linux kernel version is bumped in Buildroot with commit messages of type:
+
+    linux: bump default to version 4.9.6
+
+So you can try:
+
+    git log --grep 'linux: bump default to version'
+
+Those commits change `BR2_LINUX_KERNEL_LATEST_VERSION` in `/linux/Config.in`.
+
+You should then look up if there is a branch that supports that kernel. Staying on branches is a good idea as they will get backports, in particular ones that fix the build as newer host versions come out.
+
 ## How to add new Buildroot options?
 
     cd buildroot/output.x86_64~