linux-kernel - Re: [PATCH v3 2/3] x86/vmscape: Replace IBPB with branch history clear on exit to userspace

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Message-ID: <aQKZmoabW0M9STCa@google.com>
Date: Wed, 29 Oct 2025 15:47:54 -0700
From: Sean Christopherson <seanjc@...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>, Paolo Bonzini <pbonzini@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.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 v3 2/3] x86/vmscape: Replace IBPB with branch history
 clear on exit to userspace

On Mon, Oct 27, 2025, Pawan Gupta wrote:
> IBPB mitigation for VMSCAPE is an overkill for CPUs that are only affected
> by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides
> indirect branch isolation between guest and host userspace. But, a guest
> could still poison the branch history.
> 
> To mitigate that, use the recently added clear_bhb_long_loop() to isolate
> the branch history between guest and userspace. Add cmdline option
> 'vmscape=on' that automatically selects the appropriate mitigation based
> on the CPU.
> 
> Acked-by: David Kaplan <david.kaplan@....com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
>  Documentation/admin-guide/hw-vuln/vmscape.rst   |  8 ++++
>  Documentation/admin-guide/kernel-parameters.txt |  4 +-
>  arch/x86/include/asm/cpufeatures.h              |  1 +
>  arch/x86/include/asm/entry-common.h             | 12 +++---
>  arch/x86/include/asm/nospec-branch.h            |  2 +-
>  arch/x86/kernel/cpu/bugs.c                      | 53 ++++++++++++++++++-------
>  arch/x86/kvm/x86.c                              |  5 ++-
>  7 files changed, 61 insertions(+), 24 deletions(-)

For the KVM changes,

Acked-by: Sean Christopherson <seanjc@...gle.com>