When you are trying to start a KVM guest via libvirt on an s390x Linux installation that is running on an older version of z/VM, you might run into the problem that QEMU refuses to start with this error message:
cannot set up guest memory 's390.ram': Permission denied.
This happens because older versions of z/VM (before version 6.3) do not support the so-called “enhanced suppression on protection facility” (ESOP) yet, so QEMU has to allocate the memory for the guest with a “hack”, and this hack uses mmap(… PROT_EXEC …) for the allocation.
Now this mmap() call is not allowed by the default SELinux rules (at least not on RHEL-based systems), so QEMU fails to allocate the memory for the guest here. Turning off SELinux completely just to run a KVM guest is of course a bad idea, but fortunately there is already a SELinux boolean value called virt_use_execmem which can be used to tune the behavior here:
setsebool virt_use_execmem 1
This configuration switch has originally been introduced for running TCG guests (i.e. running QEMU without KVM), but in this case it also fixes the problem with KVM guests. Anyway, since setting this SELinux variable to 1 is also a slight decrease in security (but still better than disabling SELinux completely), you should better upgrade your z/VM to version 6.3 (or newer) or use a real LPAR for the KVM host installation instead, if that is feasible.