linux-kernel - Re: [PATCH 1/2] x86/bhi: Add BHB clearing for CPUs with larger branch history

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Message-ID: <CALMp9eRcDZoRza7pkCx_fmYZ9UZDGRAXQ_0QP=v+pMMBKx4gfg@mail.gmail.com>
Date: Thu, 25 Sep 2025 10:54:48 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	David Kaplan <david.kaplan@....com>, Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, 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 1/2] x86/bhi: Add BHB clearing for CPUs with larger branch history

On Wed, Sep 24, 2025 at 8:09 PM Pawan Gupta
<pawan.kumar.gupta@...ux.intel.com> wrote:
>
> Add a version of clear_bhb_loop() that works on CPUs with larger branch
> history table such as Alder Lake and newer. This could serve as a cheaper
> alternative to IBPB mitigation for VMSCAPE.

Yay!

Can we also use this longer loop as a BHI mitigation on (virtual)
processors with larger branch history tables that don't support
BHI_DIS_S? Today, we just use the short BHB clearing loop and call it
good.