linux-kernel - Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb

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Message-ID: <8b657ef2-d9a7-4424-987d-111beb477727@suse.com>
Date: Fri, 21 Nov 2025 14:33:38 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
 David Kaplan <david.kaplan@....com>, "H. Peter Anvin" <hpa@...or.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Sean Christopherson
 <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 Asit Mallick <asit.k.mallick@...el.com>, Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on
 newer CPUs

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@...ux.intel.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>

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