linux-kernel - Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb

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Message-ID: <20251124192156.jo3rpewj3pa7x4i2@desk>
Date: Mon, 24 Nov 2025 11:21:56 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Nikolay Borisov <nik.borisov@...e.com>, 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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on
 newer CPUs

On Fri, Nov 21, 2025 at 01:36:37PM -0800, Dave Hansen wrote:
> On 11/21/25 13:26, Pawan Gupta wrote:
> > On Fri, Nov 21, 2025 at 10:42:24AM -0800, Dave Hansen wrote:
> >> On 11/21/25 10:16, Pawan Gupta wrote:
> ...>>> Also I was preferring constants because load values from global
> variables
> >>> may also be subject to speculation. Although any speculation should be
> >>> corrected before an indirect branch is executed because of the LFENCE after
> >>> the sequence.
> >>
> >> I guess that's a theoretical problem, but it's not a practical one.
> > 
> > Probably yes. But, load from memory would certainly be slower compared to
> > immediates.
> 
> Yeah, but it's literally two bytes of data that can almost certainly be
> shoved in a cacheline that's also being read on kernel entry. I suspect
> it would be hard to show a delta between a memory load and an immediate.
> 
> I'd love to see some actual data.

You were right, the perf-tool profiling and the Unixbench results show no
meaningful difference between the two approaches. I was irrationally biased
towards immediates. Making the loop count as global.