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[Qemu-devel] [PATCH] blogposts: add post about the new check-tcg infrast

From: Alex Bennée
Subject: [Qemu-devel] [PATCH] blogposts: add post about the new check-tcg infrastructure
Date: Thu, 21 Jun 2018 19:41:16 +0100

Signed-off-by: Alex Bennée <address@hidden>
 _posts/2018-06-21-tcg-testing.md | 129 +++++++++++++++++++++++++++++++
 1 file changed, 129 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
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+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 address@hidden 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=TODO) 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. Notably the cris, lm32, mips, openrisc and xtensa targets have
+a set of tests that need a system emulator. Now we have the
+infrastructure for compiling I hope we can get support for system
+tests added fairly quickly. There will need to be some work to figure
+out a nice common way to pass results back to the build-system. For
+linux-user this is simple as all programs can simply return their exit
+code however for system emulation this is a little more involved.

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