From: Michal Hocko <mhocko@kernel.org>
To: Dave Hansen <dave@sr71.net>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org,
torvalds@linux-foundation.org, akpm@linux-foundation.org,
bp@alien8.de, ak@linux.intel.com, dave.hansen@intel.com,
dave.hansen@linux.intel.com
Subject: Re: [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none()
Date: Thu, 14 Jul 2016 08:13:56 +0200 [thread overview]
Message-ID: <20160714061355.GA24790@dhcp22.suse.cz> (raw)
In-Reply-To: <578662A7.3040409@sr71.net>
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.
Agreed!
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2016-07-14 6:14 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-08 0:19 [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum Dave Hansen
2016-07-08 0:19 ` [PATCH 1/4] x86, swap: move swap offset/type up in PTE to work around erratum Dave Hansen
2016-07-13 8:03 ` [tip:x86/mm] x86/mm: Move " tip-bot for Dave Hansen
2016-07-13 15:19 ` [PATCH 1/4] x86, swap: move " Michal Hocko
2016-07-08 0:19 ` [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none() Dave Hansen
2016-07-13 8:03 ` [tip:x86/mm] x86/mm: Ignore " tip-bot for Dave Hansen
2016-07-13 15:21 ` [PATCH 2/4] x86, pagetable: ignore " Michal Hocko
2016-07-13 15:47 ` Dave Hansen
2016-07-14 6:13 ` Michal Hocko [this message]
2016-07-08 0:19 ` [PATCH 3/4] x86: disallow running with 32-bit PTEs to work around erratum Dave Hansen
2016-07-13 8:04 ` [tip:x86/mm] x86/mm: Disallow " tip-bot for Dave Hansen
2016-07-08 0:19 ` [PATCH 4/4] x86: use pte_none() to test for empty PTE Dave Hansen
2016-07-13 8:04 ` [tip:x86/mm] x86/mm: Use " tip-bot for Dave Hansen
2016-07-13 15:18 ` [PATCH 4/4] x86: use " Michal Hocko
2016-07-13 15:23 ` Julia Lawall
2016-07-13 15:49 ` Julia Lawall
2016-07-13 16:28 ` Dave Hansen
2016-07-14 13:47 ` Vlastimil Babka
2016-07-14 14:24 ` Dave Hansen
2016-07-14 14:50 ` David Vrabel
2016-07-13 9:54 ` [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum Vlastimil Babka
-- strict thread matches above, loose matches on Subject: below --
2016-07-01 17:46 Dave Hansen
2016-07-01 17:47 ` [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none() Dave Hansen
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