Re: [PATCH v12 02/12] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Pawan Gupta]

On Wed, Jun 24, 2026 at 03:12:28PM +0300, Nikolay Borisov wrote:
> 
> 
> On 23.06.26 г. 20:33 ч., Pawan Gupta wrote:
> > As a mitigation for BHI, clear_bhb_loop() executes branches that overwrite
> > 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 use the BHI_DIS_S hardware mitigation
> > in the kernel.
> > 
> > Now with VMSCAPE (BHI variant) it is also required to isolate branch
> > history between guests and userspace. Since BHI_DIS_S only protects the
> > kernel, the newer CPUs also use IBPB.
> > 
> > A cheaper alternative to the current IBPB mitigation is clear_bhb_loop().
> > But it currently does not clear enough BHB entries to be effective on newer
> > CPUs with larger BHB. At boot, dynamically set the loop count of
> > clear_bhb_loop() such that it is effective on newer CPUs too.
> > 
> > Introduce global loop counts, initializing them with appropriate value
> > based on the hardware feature X86_FEATURE_BHI_CTRL.
> > 
> > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Acked-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> 
> Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> 
> Although AI brings up a valid argument about whether guests should be
> pessimized and fallback to the longer sequence ?

I don't disagree, but at the same time BHI mitigation for guest migration
is a different beast that should be addressed separately. A series that
adds virtual-SPEC_CTRL support is in the works. Expect the RFC to be posted
in a couple of weeks.