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: <20251030000859.ufyrqzyhvro44mol@desk>
Date: Wed, 29 Oct 2025 17:08:59 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.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 Wed, Oct 29, 2025 at 03:47:54PM -0700, Sean Christopherson wrote:
> 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>

Thank you.