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

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Message-ID: <3db1228d-66af-4f2b-8fc3-506203dddf83@intel.com>
Date: Fri, 21 Nov 2025 13:36:37 -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 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.

>> 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
> 
> Both of these look to be good options to me.
> 
> 2b. would be my first preference, because it keeps the loop counts as
> inline constants. The resulting sequence stays the same as it is today.
> 
>> 3. Store the inner/outer loop counts in variables in memory
> 
> I could be wrong, but this will likely have non-zero impact on performance.
> I am afraid to cause any regressions in BHI mitigation. That is why I
> preferred the least invasive approach in my previous attempts.

Your magic 8-ball and my crystal ball seem to be disagreeing today.

Time for science!