Re: [PATCH v14 03/13] x86/mm: add INVLPGB support code

[Dave Hansen]

On 3/4/25 08:19, Borislav Petkov wrote:
> +static inline void __invlpgb_all(unsigned long asid, unsigned long pcid,
> +				 unsigned long addr, u16 nr_pages, u8 flags)
> +{
> +	__invlpgb(asid, pcid, addr, nr_pages, 0, flags);
> +}

Why would __invlpg_all() need an 'addr' or 'nr_pages'? Shouldn't those be 0?

It's _better_ of course when it happens at a single site and it's close
to a prototype for __invlpgb(). But it's still a magic '0' that it's
impossible to make sense of without looking at the prototype.

Looking at the APM again... there really are three possible values for
ECX[31]:

 0: increment by 4k
 1: increment by 2M
 X: Don't care, no increment is going to happen

What you wrote above could actually be written:

	__invlpgb(asid, pcid, addr, nr_pages, 1, flags);

so the 0/1 is _actually_ completely random and arbitrary as far as the
spec goes.

Why does it matter?

It enables you to do sanity checking. For example, we could actually
enforce a rule that "no stride" can't be paired with any of the
per-address invalidation characteristics:

	if (stride == NO_STRIDE) {
		WARN_ON(flags & INVLPGB_FLAG_VA);
		WARN_ON(addr);
		WARN_ON(nr_pages);
	}

That's impossible if you pass a 'bool' in.

But, honestly, I'm deep into nitpick mode here. I think differentiating
the three cases is worth it, but it's also not the hill I'm going to die
on. ;)