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Thu, 18 Jun 2020 16:20:27 +0200 (CEST) Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages To: Michael Ellerman , Peter Zijlstra Cc: Benjamin Herrenschmidt , Paul Mackerras , Will Deacon , Andrew Morton , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org References: <341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu> <20200615132244.GR2531@hirez.programming.kicks-ass.net> <87wo45db8d.fsf@mpe.ellerman.id.au> <20200617143826.GJ2531@hirez.programming.kicks-ass.net> <0bb024ce-11aa-80dc-c7d8-d5acc5329f25@csgroup.eu> <87o8phchnu.fsf@mpe.ellerman.id.au> From: Christophe Leroy Message-ID: Date: Thu, 18 Jun 2020 16:19:33 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <87o8phchnu.fsf@mpe.ellerman.id.au> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr X-Rspamd-Queue-Id: BEED58E585 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Le 18/06/2020 =C3=A0 03:00, Michael Ellerman a =C3=A9crit=C2=A0: > Christophe Leroy writes: >> Le 17/06/2020 =C3=A0 16:38, Peter Zijlstra a =C3=A9crit=C2=A0: >>> On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote: >>>> Peter Zijlstra writes: >>>>> On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote: >>> >>>>>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >>>>>> +#define __HAVE_ARCH_PTEP_GET >>>>>> +static inline pte_t ptep_get(pte_t *ptep) >>>>>> +{ >>>>>> + pte_t pte =3D {READ_ONCE(ptep->pte), 0, 0, 0}; >>>>>> + >>>>>> + return pte; >>>>>> +} >>>>>> +#endif >>>>> >>>>> Would it make sense to have a comment with this magic? The casual r= eader >>>>> might wonder WTH just happened when he stumbles on this :-) >>>> >>>> I tried writing a helpful comment but it's too late for my brain to = form >>>> sensible sentences. >>>> >>>> Christophe can you send a follow-up with a comment explaining it? In >>>> particular the zero entries stand out, it's kind of subtle that thos= e >>>> entries are only populated with the right value when we write to the >>>> page table. >>> >>> static inline pte_t ptep_get(pte_t *ptep) >>> { >>> unsigned long val =3D READ_ONCE(ptep->pte); >>> /* 16K pages have 4 identical value 4K entries */ >>> pte_t pte =3D {val, val, val, val); >>> return pte; >>> } >>> >>> Maybe something like that? >> >> This should work as well. Indeed nobody cares about what's in the othe= r >> three. They are only there to ensure that ptep++ increases the ptep >> pointer by 16 bytes. Only the HW require 4 identical values, that's >> taken care of in set_pte_at() and pte_update(). >=20 > Right, but it seems less error-prone to have the in-memory > representation match what we have in the page table (well that's > in-memory too but you know what I mean). >=20 >> So we should use the most efficient. Thinking once more, maybe what yo= u >> propose is the most efficient as there is no need to load another >> register with value 0 in order to write it in the stack. >=20 > On 64-bit I'd say it makes zero difference, the only thing that's going > to matter is the load from ptep->pte. I don't know whether that's true > on the 8xx cores though. On 8xx core, loading a register with value 0 will take one cycle unless=20 there is some bubble left by another instruction (like a load from=20 memory or a taken branch). But that's in the noise. Christophe