ensuring that probe attempts of this kind will not reveal anything useful. Modern operating systems use memory mapping to increase speed so this could lead to performance loss. Virtual machine – A Meltdown attack cannot be used to break out of a virtual machine, i.e., in fully virtualized machines guest user space can still read from guest kernel space, but not from host kernel space. The bug enables reading memory from address space represented by the same page table, meaning the bug does not work between virtual tables. That is, guest-to-host page tables are unaffected, only guest-to-same-guest or host-to-host, and of course host-to-guest since the host can already access the guest pages. This means different VMs on the same fully virtualized hypervisor cannot access each other's data, but different users on the same guest instance can access each other's data. Embedded device – Among the vulnerable chips are those designed by ARM and Intel designed for standalone and embedded devices, such as mobile phones, smart TVs, networking equipment, vehicles, hard drives, industrial control, and the like. As with all vulnerabilities, if a third party cannot run code on the device, its internal vulnerabilities remain unexploitable. For example, an ARM processor in a cellphone or Internet of Things "smart" device may be vulnerable, but the same processor used in a device that cannot download and run new code, such as a kitchen appliance or hard drive controller, is believed to not be exploitable. The specific impact depends on the implementation of the address translation mechanism in the OS and the underlying hardware architecture. The attack can reveal the content of any memory that is mapped into a user address space, even if otherwise protected. For example, before kernel page-table isolation was introduced, most versions of Linux mapped all physical memory into the address space of every user-space process; the mapped addresses are (mostly) protected, making them unreadable from user-space and accessible only when transitioned into the kernel. The existence of these mappings makes transitioning to and from the kernel faster, but is unsafe in the presence of the Meltdown vulnerability, as the contents of all physical memory (which may contain sensitive information such as passwords belonging to other processes or the kernel) can then be obtained via the above method by any unprivileged process from user-space. According to researchers, "every Intel processor that implements out-of-order execution is potentially affected, which is