Next Linux Kernel Bringing "Speculative Store Bypass Safe" For ARMv8.5

Speculative Store Bypass Safe (SSBS) is a new bit added with ARMv8.5-A for SoCs moving forward like the ARM Cortex-A76 as a means for mitigation against Spectre V4.

Queued within the ARM64/AArch64 for the upcoming Linux 4.20~5.0 cycle is detection for SSBS and the ability to advertise its presence to user-space. This Speculative Store Bypass Safe functionality relies on the ARM Linux infrastructure that's already existing for Speculative Store Bypass Disable (SSBD).

This initial ARMv8.5 SSBS code is queued in ARM64 for-next/core ahead of this next kernel cycle that will be kicking off as soon as a week from today if the rest of the Linux 4.19 kernel cycle wraps up nicely.

SSBS should help with the ARM performance while fending off Spectre Variant 4, but ARM hasn't published much documentation on this ARMv8.5 addition yet nor any formal performance metrics of the impact. ARMv8.5 also brings other additions for protecting against Spectre as we've seen on the compiler front too with new speculation restriction system registers and SSBB / PSSBB speculation barriers.

ARMv8.5 though isn't just about dealing with Spectre fallout as it also has new persistent memory instructions, memory tagging, virtualization enhancements, and new random number instructions too. The first ARMv8.5 hardware should be out in 2019.