Note that FSGSBASE support arrived ridiculously late in Linux, in Linux 5.9. It was first added to CPUs with Ivy Bridge: https://www.intel.com/content/www/us/en

Note that FSGSBASE support arrived ridiculously late in Linux, in Linux 5.9. It was first added to CPUs with Ivy Bridge: https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/guidance-enabling-fsgsbase.html https://www.intel.com/content/www/us/en/developer/articles/t... AMD support is of similar vintage. As a result of the later kernel support, it may not be for everyone to turn on. Until then, the replacement on GNU Linux is just to load %fs:0, which the x86-64 ABI requires to have the same value (or %gs:0 on i386). However, usually, for initial-exec TLS access, the address of variable is not required to be in a register. Is SEGFS/%fs-based access slower than loading the base address with RDFSBASE (which may require spilling a register) and then using base+offset access? I haven't seen such reports.