Re: [PATCH v9 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Jim Mattson]

On Tue, Apr 7, 2026 at 4:41 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 4/7/26 16:27, Jim Mattson wrote:
> > What is your proposed BHI_DIS_S override mechanism, then?
>
> Let me make sure I get this right. The desire is to:
>
> 1. Have hypervisors lie to guests about the CPU they are running on (for
>    the benefit of large/diverse migration pools)
> 2. Have guests be allowed to boot with BHI_DIS_S for performance
> 3. Have apps in those guests that care about security to opt back in to
>    BHI_DIS_S for themselves?

I just want guests on heterogeneous migration pools to properly
protect themselves from native BHI when running on host kernels at
least as far back as Linux v6.6.

To that end, I would be satisfied with using the longer BHB clearing
sequence when HYPERVISOR is true and BHI_CTRL is false.