Am I correct to assume that support for running 32-bit code on a 64-bit kernel using compatibility mode rather than virtualization and support for (presumably limited) direct system calls from VMs, bypassing any user-mode VMM (using something like Xen's trampoline page) wouldn't be accepted into seL4?
These are yet more things that I am going to want at some point and can't think of any other way to do them (besides adding them to the kernel) without significant penalties to hardware compatibility (in the case of the 32-bit support; while UX/RT will mostly be focused on 64-bit systems, it will support 32-bit Linux programs in its compatibility layer) or performance (in the case of direct kernel traps from VMs; I am planning to write a dynamic general-purpose hypervisor to go with UX/RT eventually, and adding an extra trap into the VMM on every IPC would likely add a fair bit of overhead).