A Tentative Linux Kernel Patch For Able AMD CPUs To Address Spectre V4 / SSBD

Linux kernel developer veteran Konrad Rzeszutek Wilk at Oracle has posted a set of experimental/debug patches for dealing with AMD "Speculative Store Bypass Disable" (SSBD) support for mitigating the Spectre Variant Four vulnerability.

This set of three kernel patches is for dealing with two new published bits for indicating when Speculative Store Bypass Disable (SSBD) isn't needed anymore for future generations of mitigated CPUs and the MSR to set for toggling the SSB behavior on AMD processors. The SPEC_CTL MSR 48h bit will only be present on future generations of AMD CPUs while there is also a new VIRT_SPEC_CTRL MSR for keeping the processor from speculating around older stores. Presumably the VIRT_SPEC_CTRL will be introduced in an upcoming microcode update.

Konrad has sent out the tentative patches for making use of these new bits albeit hasn't seen any actual testing yet.