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

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Message-ID: <20251121181632.czfwnfzkkebvgbye@desk>
Date: Fri, 21 Nov 2025 10:16:32 -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 08:50:17AM -0800, Dave Hansen wrote:
> On 11/21/25 08:45, Nikolay Borisov wrote:
> > OTOH: the global variable approach seems saner as in the macro you'd
> > have direct reference to them and so it will be more obvious how things
> > are setup.
> 
> Oh, yeah, duh. You don't need to pass the variables in registers. They
> could just be read directly.

IIUC, global variables would introduce extra memory loads that may slow
things down. I will try to measure their impact. I think those global
variables should be in the .entry.text section to play well with PTI.

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.