Linux 6.12 Patches Neoverse-N3 & Microsoft Azure Cobalt 100 For Speculative SSBS

We are not done yet seeing new Arm cores still impacted by the Speculative Store Bypass handling errata. Merged to Linux 6.12 on Friday was adding the speculative SSBS workaround for the Cortex-A715, Neoverse-N3, and Microsoft Azure Cobalt 100 cores.

Over the past few months we have seen more Arm CPU cores needing the speculative SBBS workaround. The issue deals with MSR writes to the SSBS (Speculative Store Bypass Safe) special purpose register not affecting subsequent speculative instructions. This issue when unmitigated can lead to speculative store bypassing for a period of time.

The workaround within the Linux kernel adds a speculation barrier after MSR writes to the SSBS register. A variety of Cortex and Neoverse cores to date have needed this workaround while the list has grown longer as of Friday. The Cortex-A715, Neoverse-N3, and Microsoft Azure Cobalt 100 are the latest where this workaround/mitigation is being applied.

This went into effect via this merge to Linux 6.12 Git on Friday and will be found in Sunday's Linux 6.12-rc2 kernel.