Re: [PATCH] x86/speculation: Add BHI_DIS support

[Peter Zijlstra]

On Thu, Jul 14, 2022 at 12:52:35PM -0700, Daniel Sneddon wrote:
> Branch History Injection (BHI) attacks can be mitigated using the
> BHI_DIS_S indirect predictor control bit located in MSR_IA32_SPEC_CTRL
> register. Set BHI_DIS in MSR_IA32_SPC_CTRL to prevent predicted
> targets of indirect branches executed in CPL0, CPL1, or CPL2 from
> being selected based on branch history from branches executed in CPL3.
> Support for this feature is enumerated by CPUID.7.2.EDX[BHI_CTRL] (bit 4).

What actual hardware will have this?

> Users wanting BHI protection can specify spectre_v2=eibrs,bhi_dis to
> enable hardware BHI protections.  On platforms where BHI protections
> are not available in the hardware revert to eibrs,retpoline
> mitigations.

AFAICT this doesn't get auto-selected; how bad is performance for this
to not be so?