Signed-off-by: Alex Bennée <alex.ben...@linaro.org> --- v2 - changes to the language for the lesser known architectures --- _posts/2018-06-21-tcg-testing.md | 139 +++++++++++++++++++++++++++++++ 1 file changed, 139 insertions(+) create mode 100644 _posts/2018-06-21-tcg-testing.md
diff --git a/_posts/2018-06-21-tcg-testing.md b/_posts/2018-06-21-tcg-testing.md new file mode 100644 index 0000000..dfc504c --- /dev/null +++ b/_posts/2018-06-21-tcg-testing.md @@ -0,0 +1,139 @@ +--- +layout: post +title: "QEMU TCG Tests" +date: 2018-06-21 10:30:00:00 +0000 +last_modified_at: 2018-06-21 10:30:00:00 +0000 +author: Alex Bennée +categories: [testing, docker, compilation, tcg] +--- + +Ever since I started working on QEMU there was a small directory +called tests/tcg that was in a perpetually broken state. It contains +tests that exercise QEMU's ability to work across architectures using +the power of the Tiny Code Generator. However as these tests needed to +be compiled for the *guest* architectures and not the *host* +architecture - known as cross-compiling - most developers never ran +them. As the tests were hardly ever built inevitably a certain amount +of bit-rot set in. + +# Cross Compilers + +In the old days cross-compilation setups were almost all hand-crafted +affairs which involved building versions of binutils, gcc and a basic +libc. If you couldn't get someone to give you a pre-built tarball it +was something you laboured through once and hopefully never had to +touch again. There were even dedicated scripts like crosstool-ng which +attempted to make the process of patching and configuring your +toolchain easier. + +While the distributions have improved their support for cross +compilers over the years there are still plenty of variations in how +they are deployed. It is hard for a project like QEMU which has to +build on a wide range of operating systems and architectures to +seamlessly use any given distributions compiler setup. However for +those with cross compilers to hand `configure` now accepts two +additional flags: + + --cross-cc-$(ARCH) + --cross-cc-flags-$(ARCH) + +With a compiler specified for each guest architecture you want to test +the build system can now build and run the tests. However for +developers that don't have cross compilers around they can now take +advantage of QEMU's docker images. + +# Enter Docker Containers + +If you work in IT you would be hard pressed not to have noticed the +hype around Docker and the concept of containerisation over the last +few years. Put simply containers allow you to define a known working +set of software that gets run in an isolated environment for a given +task. While this has many uses for QEMU it allows us to define build +environments that any developer can run without having to mess around +with their preferred host setup. + +Over the last few years QEMU's build system has been expanding the +number of docker images it supports. Most of this has been in service +of our CI testing such as [Patchew](https://patchew.org/QEMU/) and +[Shippable](https://app.shippable.com/github/qemu/qemu/dashboard) but +any developer with a docker setup can run the exact same images. For +example if you want to check your patches won't break when compiled on +a 32 bit ARM system you can run: + + make docker-test-build@debian-armhf-cross J=n + +instead of tracking down a piece of ARM hardware to actually build on. +Run `make docker` in your source tree to see the range of builds and +tests it can support. + +# make check-tcg + +With the latest work [merged into +master](https://git.qemu.org/?p=qemu.git;a=commit;h=de44c044420d1139480fa50c2d5be19223391218) we can now +take advantage of both hand configured and docker based cross +compilers to build test cases for TCG again. To run the TCG tests +after you have built QEMU: + + make check-tcg + +and the build system will build and run all the tests it can for your +configured targets. + +# Rules for tests/tcg + +So now we have the infrastructure in place to add new tests what rules +need to be followed to add new tests? + +Well the first thing to note is currently all the tests are for +linux-user versions of QEMU. This means the tests are all currently +user-space tests that have access to the Linux syscall ABI. + +Another thing to note is the tests are standalone from the rest of the +QEMU test infrastructure. To keep things simple they are compiled as +standalone "static" binaries. As the cross-compilation setup can be +quite rudimentary for some of the rarer architectures we only compile +against a standard libc. There is no support for linking to other +libraries like for example glib. Thread and maths support is part of +glibc so shouldn't be a problem. + +Finally when writing new tests consider if it really is architecture +specific or can be added to `tests/tcg/multiarch`. The multiarch tests +are re-built for every supported architecture and should be the +default place for anything explicitly testing syscalls and other +common parts of the code base. + +# What's Next + +My hope with this work is we can start adding more tests to +systematically defend functionality in linux-user. In fact I hope the +first port of call to reproducing a crash would be writing a test case +that can be added to our growing library of tests. + +Another thing that needs sorting out is getting toolchains for all of +the less common architectures. The current work relies heavily on the +excellent work of the Debian toolchain team in making multiarch +aware cross compilers available in their distribution. However QEMU +supports a lot more architectures than QEMU, some only as system +emulations. In principle supporting them is as easy as adding another +docker recipe but it might be these recipes end up having to compile +the compilers from source. + +The `tests/tcg` directory still contains a number of source files we +don't build. + +The cris and openrisc directories contain user-space tests which just +need the support of a toolchain and the relevant Makefile plumbing to +be added. + +The lm32, mips and xtensa targets have a set of tests that need a +system emulator. Aside from adding the compilers as docker images some +additional work is needed to handle the differences between plain +linux-user tests which can simply return an exit code to getting the +results from a qemu-system emulation. Some architectures have +semi-hosting support already for this while others report their test +status over a simple serial link which will need to be parsed and +handled in custom versions of the +[`run-%:`](https://git.qemu.org/?p=qemu.git;a=blob;f=tests/tcg/Makefile;h=bf064153900a438e4ad8e2d79eaaac8a27d66062;hb=HEAD#l95) +rule. + + -- 2.17.1
