linux-kernel - Re: [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud

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Date:	Wed, 13 Jul 2016 08:47:51 -0700
From:	Dave Hansen <dave@...1.net>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	bp@...en8.de, ak@...ux.intel.com, dave.hansen@...el.com,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none()

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.

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.