1. A method for enforcing deterministic execution of a thread of a guest operating system (OS) running in a virtual machine (VM) by a virtual machine monitor (VMM) on a host computer having two physical processors, the method comprising: receiving from the guest OS of the VM, by the VM, a first mapping and associated permissions between a virtual address space of the thread of the guest OS and a virtual address space of the VM; installing, by the VM, the first mapping and associated permissions into a virtual memory management unit (MMU) of a virtual processor of the VM; determining a second mapping and associated permissions between the virtual address space of the thread of the guest OS and a physical address space of the host computer, the second mapping and associated permissions being indicative of a policy that enforces a deterministic order of execution for a first instruction stream of the guest OS relative to a second instruction stream of the guest OS, the first instruction stream executing on a first physical processor of the two physical processors concurrently with the second instruction stream executing on a second physical processor of the two physical processors; installing, by the VMM, a third mapping and associated permissions into a physical MMU of the host computer that establishes memory access permissions generated by the VMM and specific to the physical MMU's associated physical processor, the third mapping and associated permissions being a composition of the first mapping and associated permissions and the second mapping and associated permissions; and in response to determining that the thread of the guest OS is attempting to read from or write to a memory page of the physical address space in violation of the third mapping and associated permissions, preventing the thread of the guest OS from reading from or writing to the memory page.