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

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Message-ID: <e9678dd1-7989-4201-8549-f06f6636274b@suse.com>
Date: Fri, 21 Nov 2025 18:45:54 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Dave Hansen <dave.hansen@...el.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.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>
Cc: 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 18:40, Dave Hansen wrote:
> On 11/19/25 22:18, Pawan Gupta wrote:
>> -	CLEAR_BHB_LOOP_SEQ 5, 5
>> +	/* loop count differs based on CPU-gen, see Intel's BHI guidance */
>> +	ALTERNATIVE (CLEAR_BHB_LOOP_SEQ 5, 5),  \
>> +		    __stringify(CLEAR_BHB_LOOP_SEQ 12, 7), X86_FEATURE_BHI_CTRL
> 
> There are a million ways to skin this cat. But I'm not sure I really
> like the end result here. It seems a little overkill to use ALTERNATIVE
> to rewrite a whole sequence just to patch two constants in there.
> 
> What if the CLEAR_BHB_LOOP_SEQ just took its inner and outer loop counts
> as register arguments? Then this would look more like:
> 
> 	ALTERNATIVE "mov  $5, %rdi; mov $5, %rsi",
> 		    "mov $12, %rdi; mov $7, %rsi",
> 	...
> 
> 	CLEAR_BHB_LOOP_SEQ
> 
> Or, even global variables:
> 
> 	mov outer_loop_count(%rip), %rdi
> 	mov inner_loop_count(%rip), %rsi

nit: FWIW I find this rather tacky, because the way the registers are 
being used (although they do follow the x86-64 calling convention) is 
obfuscated in the macro itself.

> 
> and then have some C code somewhere that does:
> 
> 	if (cpu_feature_enabled(X86_FEATURE_BHI_CTRL)) {
> 		outer_loop_count = 5;
> 		inner_loop_count = 5;
> 	} else {
> 		outer_loop_count = 12;
> 		inner_loop_count = 7;
> 	}

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.

<snip>