Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Nikolay Borisov]

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>