Comment 11 for bug 906922

Right, so this is really down to qemu not being very good at handling mmap() in the 32-bit-guest-on-64-bit-host case -- it tends to fail mmap() even when there's more address space available because it hasn't managed to get the host kernel to allocate it within the 32-bit region of the address space the guest can see.

We're currently tossing around the idea of making (the equivalent of) that reserved-va setting the default upstream.