linux-kernel - Re: [PATCH v4 09/11] x86/vmscape: Deploy BHB clearing mitigation

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Message-ID: <20251121182935.klm43ygvtoni4y7a@desk>
Date: Fri, 21 Nov 2025 10:29:35 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: 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>,
	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 09/11] x86/vmscape: Deploy BHB clearing mitigation

On Fri, Nov 21, 2025 at 04:18:09PM +0200, Nikolay Borisov wrote:
> 
> 
> On 11/20/25 08:19, Pawan Gupta wrote:
> > IBPB mitigation for VMSCAPE is an overkill on 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. However, branch
> > history from guest may also influence the indirect branches in host
> > userspace.
> > 
> > To mitigate the BHI aspect, use clear_bhb_loop().
> > 
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> 
> <snip>
> 
> > @@ -3278,6 +3290,9 @@ static void __init vmscape_apply_mitigation(void)
> >   {
> >   	if (vmscape_mitigation == VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER)
> >   		static_call_update(vmscape_predictor_flush, write_ibpb);
> > +	else if (vmscape_mitigation == VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER &&
> > +		 IS_ENABLED(CONFIG_X86_64))
> 
> why the x86_64 dependency ?

BHI sequence mitigation is only supported in 64-bit mode. I will add a
comment. Looking at it again, I realized that 64-bit check should be in
vmscape_select_mitigation(), otherwise we report incorrectly on 32-bit.

> > +		static_call_update(vmscape_predictor_flush, clear_bhb_loop);
> >   }
> >   #undef pr_fmt
> > @@ -3369,6 +3384,7 @@ void cpu_bugs_smt_update(void)
> >   		break;
> >   	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
> >   	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
> > +	case VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER:
> >   		/*
> >   		 * Hypervisors can be attacked across-threads, warn for SMT when
> >   		 * STIBP is not already enabled system-wide.
> > 
>