Documentation/Debugging with Valgrind

Valgrind works much like QEMU's TCG mode except it has a focus on debugging and testing. You can use it to detect memory corruption and leaks within QEMU.

KVM Based QEMU

valgrind's memcheck tool can be used fine with KVM-based QEMU. To avoid false positives some considerations have to be made regarding KVM ioctls when writing code. Newer versions of valgrind have already some annotations for several kvm ioctls and newer versions of QEMU have patches to quiesce false positives.

ioctl handling

All ioctls have to either follow the _IO[R][W] logic or need specific handlers in valgrind.

Valgrind has a default handler for unknown ioctls that parses the _IO[R][W] definition. So in case of an _IOxW ioctl with a structure as parameter, it will complain about undefined parameters if any byte of that structure is not pre-initialized. This is also true for padding and compiler alignment. The most simple "fix" is a designated initializer, e.g.

   struct kvm_blah blah = {
       .field1 = 1,
};

TCG Based QEMU

You will need to use the --smc-check=all (or --smc-check=all-non-file with newer valgrind versions) option to instruct valgrind to detect self-modifying code which TCG makes extensive use of.

softmmu debugging

Valgrind can be simply run by putting the it in-front on your qemu-system invocation

linux-user debugging

This is potentially complicated by the fact that a qemu-linux-user instance is usually run in a guest chroot. However with multi-arch systems you can bind mount the required host paths into your chroot so valgrind (and non-static qemu binaries) can run.