From 71d673bac48f43a2e38f5e1e4f94b10da15b7cee Mon Sep 17 00:00:00 2001
From: Ciro Santilli <ciro.santilli@gmail.com>
Date: Sun, 1 Oct 2017 19:26:24 +0100
Subject: [PATCH] Look into walking kernel start point, failure

---
 README.md | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/README.md b/README.md
index f979809..e1eb805 100644
--- a/README.md
+++ b/README.md
@@ -232,12 +232,16 @@ See also:
 - <https://superuser.com/questions/488263/problems-switching-to-qemu-control-panel-with-nographics>
 - <https://superuser.com/questions/1087859/how-to-quit-the-qemu-monitor-when-not-using-a-gui/1211516#1211516>
 
-TODO: Ctrl + C kills the emulator, it is not sent to guest processes. See:
+Limitations:
 
-- <https://github.com/cloudius-systems/osv/issues/49>
-- <https://unix.stackexchange.com/questions/167165/how-to-pass-ctrl-c-in-qemu>
+-   TODO: Ctrl + C kills the emulator, it is not sent to guest processes. See:
 
-This is however fortunate when running QEMU with GDB, as the Ctrl + C reaches GDB and breaks.
+    - <https://github.com/cloudius-systems/osv/issues/49>
+    - <https://unix.stackexchange.com/questions/167165/how-to-pass-ctrl-c-in-qemu>
+
+    This is however fortunate when running QEMU with GDB, as the Ctrl + C reaches GDB and breaks.
+
+-   Very early kernel messages such as `early console in extract_kernel` only show on the GUI, since at such early stages, not even the serial has been setup.
 
 ## Automatic startup commands
 
@@ -315,13 +319,7 @@ See also: <http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-ke
 
 If you are using text mode:
 
-    ./runqemu -d -n
-
-QEMU cannot be put on the background of the current shell, so you will need to open a separate terminal and run:
-
-    ./rungdb
-
-`O=0` is however an impossible dream, `O=2` being the default: <https://stackoverflow.com/questions/29151235/how-to-de-optimize-the-linux-kernel-to-and-compile-it-with-o0> So get ready for some weird jumps, and `<value optimized out>` fun. Why, Linux, why.
+`O=0` is an impossible dream, `O=2` being the default: <https://stackoverflow.com/questions/29151235/how-to-de-optimize-the-linux-kernel-to-and-compile-it-with-o0> So get ready for some weird jumps, and `<value optimized out>` fun. Why, Linux, why.
 
 ### Kernel module debugging
 
@@ -591,9 +589,11 @@ Best attempt so far:
 
     time ./runqemu  -n -e 'init=/init_poweroff.out' -- -trace exec_tb,file=trace && \
       time ./qemu/scripts/simpletrace.py qemu/trace-events trace >trace.txt && \
-      wc -l trace.txt
+      wc -l trace.txt &&
+      sed '/0x1000000/q' trace.txt >trace-boot.txt &&
+      wc -l trace-boot.txt &&
 
-Parameter notes:
+Notes:
 
 -   `-n` is a good idea to reduce the chances that you send unwanted non-deterministic mouse or keyboard clicks to the VM.
 
@@ -603,6 +603,24 @@ Parameter notes:
 
     And it becomes even worse if you try to `-net none` as recommended in the 2.7 `replay.txt` docs, because then `ifup` waits for 15 seconds before giving up as per `/etc/network/interfaces` line `wait-delay 15`.
 
+-   `0x1000000` is the address where QEMU puts the Linux kernel at with `-kernel` in x86.
+
+    It can be found from:
+
+        readelf -e buildroot/output.x86_64~/build/linux-*/vmlinux | grep Entry
+
+    TODO confirm further. If I try to break there with:
+
+        ./rungdb *0x1000000
+
+    but I have no corresponding source line. Also note that this line is not actually the first line, since the kernel messages such as `early console in extract_kernel` have already shown on screen at that point. This does not break at all:
+
+        ./rungdb extract_kernel
+
+    It only appears once on every log I've seen so far, checked with `grep 0x1000000 trace.txt`
+
+    Then when we count the instructions that run before the kernel entry point, there is only about 100k instructions, which is insignificant compared to the kernel boot itself.
+
 This works because we have already done the following with QEMU:
 
 -   `./configure --enable-trace-backends=simple`. This logs in a binary format to the trace file.