Merge #239

239: QEMU debugging updates r=adamgreig a=blelem

Addressing a few issues:
Fixes #237, Fixes #234, Fixes #217 

The biggest issue this PR is addressing is correcting the "debugging QEMU with GDB" section that really didn't work with latest ARM toolchain (on my setup), because of two things:
 - gdb didn't map the rust libcore functions properly.  For example, after connecting to a QEMU instance, it did not show the current PC to be on the Reset function, but some random method.  
 - gdb didn't step over (`next`), after breaking at `main`. It just ran to the end of the program.

The user is now instructed to disable the LDD and use the GNU ARM linker instead, which addresses the issues with the mapping.

And instead of doing a simple, `break main`  which did not allow the user to step over after hitting the breakpoint, the user is now guided to do a more complicated,
`break hello::__cortex_m_rt_main`. After hitting that breakpoint, the `next` and `step` commands are working as one would expect.

I am no expert in embedded rust, so the proposed solution may not be optimal, but I think they are better than the current instructions provided by the book which are not working at all on my setup. (Windows 10, with latest versions of QEMU, ARM toolchain, Rust) 



Co-authored-by: Berthier Lemieux <berthier.lemieux@u-blox.com>

Author: bors[bot]

src/intro/install/verify.md

@@ -17,7 +17,7 @@ ST-LINK header is circled in red.
 Now run the following command:
 
 ``` console
-$ openocd -f interface/stlink-v2-1.cfg -f target/stm32f3x.cfg
+$ openocd -f interface/stlink.cfg -f target/stm32f3x.cfg
 ```
 
 You should get the following output and the program should block the console:

src/intro/install/windows.md

@@ -16,13 +16,13 @@ GNU gdb (GNU Tools for Arm Embedded Processors 7-2018-q2-update) 8.1.0.20180315-
 
 ## OpenOCD
 
-There's no official binary release of OpenOCD for Windows but there are unofficial releases
-available [here][openocd]. Grab the 0.10.x zipfile and extract it somewhere on your drive (I
-recommend `C:\OpenOCD` but with the drive letter that makes sense to you) then update your `%PATH%`
-environment variable to include the following path: `C:\OpenOCD\bin` (or the path that you used
-before).
+There's no official binary release of OpenOCD for Windows but if you're not in the mood to compile
+it yourself, the xPack project provides a binary distribution, [here][openocd]. Follow the
+provided installation instructions. Then update your `%PATH%` environment variable to
+include the path where the binaries were installed. (`C:\Users\USERNAME\AppData\Roaming\xPacks\@xpack-dev-tools\openocd\0.10.0-13.1\.content\bin\`,
+if you've been using the easy install) 
 
-[openocd]: https://github.com/gnu-mcu-eclipse/openocd/releases
+[openocd]: https://xpack.github.io/openocd/
 
 Verify that OpenOCD is in your `%PATH%` with:
 

src/start/qemu.md

@@ -16,7 +16,10 @@ substitutions.
 ## Creating a non standard Rust program
 
 We'll use the [`cortex-m-quickstart`] project template to generate a new
-project from it.
+project from it. The created project will contain a barebone application: a good
+starting point for a new embedded rust application. In addition, the project will
+contain an `examples` directory, with several separate applications, highlighting
+some of the key embedded rust functionality. 
 
 [`cortex-m-quickstart`]: https://github.com/rust-embedded/cortex-m-quickstart
 
@@ -151,8 +154,14 @@ target = "thumbv7m-none-eabi"    # Cortex-M3
 ```
 
 To cross compile for the Cortex-M3 architecture we have to use
-`thumbv7m-none-eabi`. This compilation target has been set as the default so the
-two commands below do the same:
+`thumbv7m-none-eabi`. That target is not automatically installed when installing
+the Rust toolchain, it would now be a good time to add that target to the toolchain,
+if you haven't done it yet:
+``` console
+$ rustup target add thumbv7m-none-eabi
+```
+ Since the `thumbv7m-none-eabi` compilation target has been set as the default in 
+ your `.cargo/config` file, the two commands below do the same:
 
 ```console
 cargo build --target thumbv7m-none-eabi
@@ -201,8 +210,6 @@ ELF Header:
 
 `cargo-size` can print the size of the linker sections of the binary.
 
-> **NOTE** this output assumes that rust-embedded/cortex-m-rt#111 has been
-> merged
 
 ```console
 cargo size --bin app --release -- -A
@@ -480,20 +487,45 @@ Reset () at $REGISTRY/cortex-m-rt-0.6.1/src/lib.rs:473
 473     pub unsafe extern "C" fn Reset() -> ! {
 ```
 
+
 You'll see that the process is halted and that the program counter is pointing
 to a function named `Reset`. That is the reset handler: what Cortex-M cores
 execute upon booting.
 
+>  Note that on some setup, instead of displaying the line `Reset () at $REGISTRY/cortex-m-rt-0.6.1/src/lib.rs:473` as shown above, gdb may print some warnings like : 
+>
+>`core::num::bignum::Big32x40::mul_small () at src/libcore/num/bignum.rs:254`
+> `    src/libcore/num/bignum.rs: No such file or directory.`
+> 
+> That's a known glitch. You can safely ignore those warnings, you're most likely at Reset(). 
+
+
 This reset handler will eventually call our main function. Let's skip all the
-way there using a breakpoint and the `continue` command:
+way there using a breakpoint and the `continue` command. To set the breakpoint, let's first take a look where we would like to break in our code, with the `list` command.
 
 ```console
-break main
+list main
 ```
+This will show the source code, from the file examples/hello.rs. 
 
 ```text
-Breakpoint 1 at 0x400: file examples/panic.rs, line 29.
+6       extern crate panic_halt;
+7
+8       use cortex_m_rt::entry;
+9       use cortex_m_semihosting::{debug, hprintln};
+10
+11      #[entry]
+12      fn main() -> ! {
+13          hprintln!("Hello, world!").unwrap();
+14
+15          // exit QEMU
+```
+We would like to add a breakpoint just before the "Hello, world!", which is on line 13. We do that with the `break` command:
+
+```console
+break 13
 ```
+We can now instruct gdb to run up to our main function, with the `continue` command:
 
 ```console
 continue
@@ -502,8 +534,8 @@ continue
 ```text
 Continuing.
 
-Breakpoint 1, main () at examples/hello.rs:17
-17          let mut stdout = hio::hstdout().unwrap();
+Breakpoint 1, hello::__cortex_m_rt_main () at examples\hello.rs:13
+13          hprintln!("Hello, world!").unwrap();
 ```
 
 We are now close to the code that prints "Hello, world!". Let's move forward
@@ -514,15 +546,7 @@ next
 ```
 
 ```text
-18          writeln!(stdout, "Hello, world!").unwrap();
-```
-
-```console
-next
-```
-
-```text
-20          debug::exit(debug::EXIT_SUCCESS);
+16          debug::exit(debug::EXIT_SUCCESS);
 ```
 
 At this point you should see "Hello, world!" printed on the terminal that's