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

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Message-ID: <e99150f3-62d4-4155-a323-2d81c1d6d47d@intel.com>
Date: Fri, 21 Nov 2025 10:42:24 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.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 11/21/25 10:16, Pawan Gupta wrote:
> 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.

Really? I didn't look exhaustively, but CLEAR_BRANCH_HISTORY seems to
get called pretty close to where the assembly jumps into C. Long after
we're running on the kernel CR3.

> 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.

So I think we have 4-ish options at this point:

1. Generate the long and short sequences independently and in their
   entirety and ALTERNATIVE between them (the original patch)
2. Store the inner/outer loop counts in registers and:
  2a. Load those registers from variables
  2b. Load them from ALTERNATIVES
3. Store the inner/outer loop counts in variables in memory