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

Nikolay Borisov
2025-11-21T12:33:41Z

On 11/20/25 08:18, Pawan Gupta 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.
>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>