Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Jim Mattson]

On Wed, Nov 19, 2025 at 10:19 PM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
>
> As a mitigation for BHI, clear_bhb_loop() executes branches that overwrites
> the Branch History Buffer (BHB). On Alder Lake and newer parts this
> sequence is not sufficient because it doesn't clear enough entries. This
> was not an issue because these CPUs have a hardware control (BHI_DIS_S)
> that mitigates BHI in kernel.
>
> BHI variant of VMSCAPE requires isolating branch history between guests and
> userspace. Note that there is no equivalent hardware control for userspace.
> To effectively isolate branch history on newer CPUs, clear_bhb_loop()
> should execute sufficient number of branches to clear a larger BHB.
>
> Dynamically set the loop count of clear_bhb_loop() such that it is
> effective on newer CPUs too. Use the hardware control enumeration
> X86_FEATURE_BHI_CTRL to select the appropriate loop count.

I didn't speak up earlier, because I have always considered the change
in MAXPHYADDR from ICX to SPR a hard barrier for virtual machines
masquerading as a different platform. Sadly, I am now losing that
battle. :(

If a heterogeneous migration pool includes hosts with and without
BHI_CTRL, then BHI_CTRL cannot be advertised to a guest, because it is
not possible to emulate BHI_DIS_S on a host that doesn't have it.
Hence, one cannot derive the size of the BHB from the existence of
this feature bit.

I think we need an explicit CPUID bit that a hypervisor can set to
indicate that the underlying hardware might be SPR or later.