linux-kernel - Re: [PATCH v4 01/11] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb

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Message-ID: <20251120165600.tpxvntu6rv7c34xd@desk>
Date: Thu, 20 Nov 2025 08:56:00 -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 01/11] x86/bhi: x86/vmscape: Move LFENCE out of
 clear_bhb_loop()

On Thu, Nov 20, 2025 at 06:15:32PM +0200, Nikolay Borisov wrote:
> 
> 
> On 11/20/25 08:17, Pawan Gupta wrote:
> > Currently, BHB clearing sequence is followed by an LFENCE to prevent
> > transient execution of subsequent indirect branches prematurely. However,
> > LFENCE barrier could be unnecessary in certain cases. For example, when
> > kernel is using BHI_DIS_S mitigation, and BHB clearing is only needed for
> > userspace. In such cases, LFENCE is redundant because ring transitions
> > would provide the necessary serialization.
> > 
> > Below is a quick recap of BHI mitigation options:
> > 
> >    On Alder Lake and newer
> > 
> >    - BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
> >                 performance overhead.
> >    - Long loop: Alternatively, longer version of BHB clearing sequence
> > 	       on older processors can be used to mitigate BHI. This
> > 	       is not yet implemented in Linux.
> 
> I find this description of the Long loop on "ALder lake and newer" somewhat
> confusing, as you are also referring "older processors". Shouldn't the
> longer sequence bet moved under "On older CPUs" heading? Or perhaps it must
> be expanded to say that the long sequence could work on Alder Lake and newer
> CPUs as well as on older cpus?

Ya, it needs to be rephrased. Would dropping "on older processors" help?

    - Long loop: Alternatively, longer version of BHB clearing sequence
		 can be used to mitigate BHI. This is not yet implemented
		 in Linux.

> > 
> >    On older CPUs
> > 
> >    - Short loop: Clears BHB at kernel entry and VMexit.

And also talk about "Long loop" effectiveness here:

    On older CPUs

    - Short loop: Clears BHB at kernel entry and VMexit. The "Long loop"
		  is effective on older CPUs as well, but should be avoided
		  because of unnecessary overhead.
> <snip>