Re: [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none()

From: Michal Hocko
Date: Thu Jul 14 2016 - 02:14:10 EST

On Wed 13-07-16 08:47:51, Dave Hansen wrote:
> On 07/13/2016 08:21 AM, Michal Hocko wrote:
> >> > This adds a tiny amount of overhead to all pte_none() checks.
> >> > I doubt we'll be able to measure it anywhere.
> > It would be better to introduce the overhead only for the affected
> > cpu models but I guess this is also acceptable. Would it be too
> > complicated to use alternatives for that?
> The patch as it stands ends up doing a one-instruction change in
> pte_none(). It goes from
> 64c8: 48 85 ff test %rdi,%rdi
> to
> 64a8: 48 f7 c7 9f ff ff ff test $0xffffffffffffff9f,%rdi
> So it essentially eats 4 bytes of icache more than it did before. But,
> it's the same number of instructions, and I can't imagine that the CPU
> will have any more trouble with a test against an immediate than a test
> against 0.

I see. Thanks for the clarification.

> We could theoretically do alternatives for this, but we would at *best*
> end up with 4 bytes of noops. So, unless the processor likes decoding 4
> noops better than 4 bytes of immediate as part of an instruction, we'll
> not win anything. *Plus* the ugliness of the assembly that we'll need
> to have the compiler guarantee that the PTE ends up in %rdi.

Michal Hocko