under Intel 64 architecture, and under Alpha, an address space number (ASN). This is because selective flushing enables the TLB behavior crucial to the exploit to be isolated across processes, without constantly flushing the entire TLB the primary reason for the cost of mitigation. On 4 January 2018, Google detailed a new technique on their security blog called "Retpoline" (a portmanteau of return and trampoline) which can overcome the Spectre vulnerability with a negligible amount of processor overhead. It involves compiler-level steering of indirect branches towards a different target that does not result in a vulnerable speculative out-of-order execution taking place. While it was developed for the x86 instruction set, Google engineers believe the technique is transferable to other processors as well. On 25 January 2018, the current status and possible future considerations in solving the Meltdown and Spectre vulnerabilities were presented. In March 2018, Intel announced that they had developed hardware fixes for Meltdown and Spectre-V2 only, but not Spectre-V1. The vulnerabilities were mitigated by a new partitioning system that improves process and privilege-level separation. On 8 October 2018, Intel is reported to have added hardware and firmware mitigations regarding Spectre and Meltdown vulnerabilities to its Coffee Lake-R processors and onwards. On 18 October 2018, MIT researchers suggested a new mitigation approach, called DAWG (Dynamically Allocated Way Guard), which may promise better security without compromising performance. On 16 April 2019, researchers from UC San Diego and University of Virginia proposed Context-Sensitive Fencing, a microcode-based defense mechanism that surgically injects fences into the dynamic execution stream, protecting against a number of Spectre variants at just 8% degradation in performance. Linux When Intel announced that Spectre mitigation can be switched on as a "security feature" instead of being an always-on bugfix, Linux creator Linus Torvalds called the patches "complete and utter garbage". Ingo Molnár then suggested the use of function tracing machinery in the Linux kernel to fix Spectre without Indirect Branch Restricted Speculation (IBRS) microcode support. This would, as a result, only have a performance impact on processors based on Intel Skylake and newer architecture. This ftrace and retpoline-based machinery was incorporated into Linux 4.15 of January 2018. The Linux kernel provides a sysfs interface to enumerate the current status of the system regarding Spectre in /sys/devices/system/cpu/vulnerabilities/ Microsoft Windows On 2 March 2019, Microsoft is reported to have released an important Windows 10 (v1809) software mitigation to the Spectre v2