From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
"Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: "linux-ia64@vger.kernel.org" <linux-ia64@vger.kernel.org>,
"linux-sh@vger.kernel.org" <linux-sh@vger.kernel.org>,
"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
"linux-riscv@lists.infradead.org"
<linux-riscv@lists.infradead.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
"linux-hexagon@vger.kernel.org" <linux-hexagon@vger.kernel.org>,
"linux-csky@vger.kernel.org" <linux-csky@vger.kernel.org>,
Christoph Hellwig <hch@infradead.org>,
"geert@linux-m68k.org" <geert@linux-m68k.org>,
"linux-snps-arc@lists.infradead.org"
<linux-snps-arc@lists.infradead.org>,
"linux-xtensa@linux-xtensa.org" <linux-xtensa@linux-xtensa.org>,
Arnd Bergmann <arnd@arndb.de>,
"linux-um@lists.infradead.org" <linux-um@lists.infradead.org>,
"linux-m68k@lists.linux-m68k.org"
<linux-m68k@lists.linux-m68k.org>,
"openrisc@lists.librecores.org" <openrisc@lists.librecores.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-parisc@vger.kernel.org" <linux-parisc@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-alpha@vger.kernel.org" <linux-alpha@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT
Date: Wed, 2 Mar 2022 15:21:22 +0530 [thread overview]
Message-ID: <9caa90f5-c10d-75dd-b403-1388b7a3d296@arm.com> (raw)
In-Reply-To: <52866c88-59f9-2d1c-6f5a-5afcaf23f2bb@csgroup.eu>
On 3/2/22 12:35 PM, Christophe Leroy wrote:
>
>
> Le 02/03/2022 à 04:22, Anshuman Khandual a écrit :
>>
>>
>> On 3/1/22 1:46 PM, Christophe Leroy wrote:
>>>
>>>
>>> Le 01/03/2022 à 01:31, Russell King (Oracle) a écrit :
>>>> On Tue, Mar 01, 2022 at 05:30:41AM +0530, Anshuman Khandual wrote:
>>>>> On 2/28/22 4:27 PM, Russell King (Oracle) wrote:
>>>>>> On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote:
>>>>>>> This defines and exports a platform specific custom vm_get_page_prot() via
>>>>>>> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX
>>>>>>> macros can be dropped which are no longer needed.
>>>>>>
>>>>>> What I would really like to know is why having to run _code_ to work out
>>>>>> what the page protections need to be is better than looking it up in a
>>>>>> table.
>>>>>>
>>>>>> Not only is this more expensive in terms of CPU cycles, it also brings
>>>>>> additional code size with it.
>>>>>>
>>>>>> I'm struggling to see what the benefit is.
>>>>>
>>>>> Currently vm_get_page_prot() is also being _run_ to fetch required page
>>>>> protection values. Although that is being run in the core MM and from a
>>>>> platform perspective __SXXX, __PXXX are just being exported for a table.
>>>>> Looking it up in a table (and applying more constructs there after) is
>>>>> not much different than a clean switch case statement in terms of CPU
>>>>> usage. So this is not more expensive in terms of CPU cycles.
>>>>
>>>> I disagree.
>>>
>>> So do I.
>>>
>>>>
>>>> However, let's base this disagreement on some evidence. Here is the
>>>> present 32-bit ARM implementation:
>>>>
>>>> 00000048 <vm_get_page_prot>:
>>>> 48: e200000f and r0, r0, #15
>>>> 4c: e3003000 movw r3, #0
>>>> 4c: R_ARM_MOVW_ABS_NC .LANCHOR1
>>>> 50: e3403000 movt r3, #0
>>>> 50: R_ARM_MOVT_ABS .LANCHOR1
>>>> 54: e7930100 ldr r0, [r3, r0, lsl #2]
>>>> 58: e12fff1e bx lr
>>>>
>>>> That is five instructions long.
>>>
>>> On ppc32 I get:
>>>
>>> 00000094 <vm_get_page_prot>:
>>> 94: 3d 20 00 00 lis r9,0
>>> 96: R_PPC_ADDR16_HA .data..ro_after_init
>>> 98: 54 84 16 ba rlwinm r4,r4,2,26,29
>>> 9c: 39 29 00 00 addi r9,r9,0
>>> 9e: R_PPC_ADDR16_LO .data..ro_after_init
>>> a0: 7d 29 20 2e lwzx r9,r9,r4
>>> a4: 91 23 00 00 stw r9,0(r3)
>>> a8: 4e 80 00 20 blr
>>>
>>>
>>>>
>>>> Please show that your new implementation is not more expensive on
>>>> 32-bit ARM. Please do so by building a 32-bit kernel, and providing
>>>> the disassembly.
>>>
>>> With your series I get:
>>>
>>> 00000000 <vm_get_page_prot>:
>>> 0: 3d 20 00 00 lis r9,0
>>> 2: R_PPC_ADDR16_HA .rodata
>>> 4: 39 29 00 00 addi r9,r9,0
>>> 6: R_PPC_ADDR16_LO .rodata
>>> 8: 54 84 16 ba rlwinm r4,r4,2,26,29
>>> c: 7d 49 20 2e lwzx r10,r9,r4
>>> 10: 7d 4a 4a 14 add r10,r10,r9
>>> 14: 7d 49 03 a6 mtctr r10
>>> 18: 4e 80 04 20 bctr
>>> 1c: 39 20 03 15 li r9,789
>>> 20: 91 23 00 00 stw r9,0(r3)
>>> 24: 4e 80 00 20 blr
>>> 28: 39 20 01 15 li r9,277
>>> 2c: 91 23 00 00 stw r9,0(r3)
>>> 30: 4e 80 00 20 blr
>>> 34: 39 20 07 15 li r9,1813
>>> 38: 91 23 00 00 stw r9,0(r3)
>>> 3c: 4e 80 00 20 blr
>>> 40: 39 20 05 15 li r9,1301
>>> 44: 91 23 00 00 stw r9,0(r3)
>>> 48: 4e 80 00 20 blr
>>> 4c: 39 20 01 11 li r9,273
>>> 50: 4b ff ff d0 b 20 <vm_get_page_prot+0x20>
>>>
>>>
>>> That is definitely more expensive, it implements a table of branches.
>>
>> Okay, will split out the PPC32 implementation that retains existing
>> table look up method. Also planning to keep that inside same file
>> (arch/powerpc/mm/mmap.c), unless you have a difference preference.
>
> My point was not to get something specific for PPC32, but to amplify on
> Russell's objection.
>
> As this is bad for ARM and bad for PPC32, do we have any evidence that
> your change is good for any other architecture ?
>
> I checked PPC64 and there is exactly the same drawback. With the current
> implementation it is a small function performing table read then a few
> adjustment. After your change it is a bigger function implementing a
> table of branches.
I am wondering if this would not be the case for any other switch case
statement on the platform ? Is there something specific/different just
on vm_get_page_prot() implementation ? Are you suggesting that switch
case statements should just be avoided instead ?
>
> So, as requested by Russell, could you look at the disassembly for other
> architectures and show us that ARM and POWERPC are the only ones for
> which your change is not optimal ?
But the primary purpose of this series is not to guarantee optimized
code on platform by platform basis, while migrating from a table based
look up method into a switch case statement.
But instead, the purposes is to remove current levels of unnecessary
abstraction while converting a vm_flags access combination into page
protection. The switch case statement for platform implementation of
vm_get_page_prot() just seemed logical enough. Christoph's original
suggestion patch for x86 had the same implementation as well.
But if the table look up is still better/preferred method on certain
platforms like arm or ppc32, will be happy to preserve that.
- Anshuman
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2022-03-02 9:53 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-28 10:47 [PATCH V3 00/30] mm/mmap: Drop protection_map[] and platform's __SXXX/__PXXX requirements Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 01/30] mm/debug_vm_pgtable: Drop protection_map[] usage Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 02/30] mm/mmap: Clarify protection_map[] indices Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 03/30] mm/mmap: Add new config ARCH_HAS_VM_GET_PAGE_PROT Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 04/30] powerpc/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT Anshuman Khandual
2022-03-02 5:23 ` Michael Ellerman
2022-02-28 10:47 ` [PATCH V3 05/30] arm64/mm: " Anshuman Khandual
2022-03-03 15:28 ` Catalin Marinas
2022-03-09 11:31 ` Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 06/30] sparc/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 07/30] mips/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 08/30] m68k/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 09/30] arm/mm: " Anshuman Khandual
2022-02-28 10:57 ` Russell King (Oracle)
2022-02-28 13:49 ` Geert Uytterhoeven
2022-03-01 0:00 ` Anshuman Khandual
2022-03-01 0:31 ` Russell King (Oracle)
2022-03-01 8:16 ` Christophe Leroy
2022-03-02 3:22 ` Anshuman Khandual
2022-03-02 7:05 ` Christophe Leroy
2022-03-02 9:51 ` Anshuman Khandual [this message]
2022-03-02 10:05 ` Geert Uytterhoeven
2022-03-02 11:06 ` Anshuman Khandual
2022-03-02 11:14 ` Geert Uytterhoeven
2022-03-09 11:33 ` Anshuman Khandual
2022-03-02 11:19 ` Russell King (Oracle)
2022-03-02 3:15 ` Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 10/30] x86/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 11/30] mm/mmap: Drop protection_map[] Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 12/30] mm/mmap: Drop arch_filter_pgprot() Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 13/30] mm/mmap: Drop arch_vm_get_page_pgprot() Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 14/30] s390/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 15/30] riscv/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 16/30] alpha/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 17/30] sh/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 18/30] arc/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 19/30] csky/mm: " Anshuman Khandual
2022-03-01 14:00 ` Guo Ren
2022-02-28 10:47 ` [PATCH V3 20/30] xtensa/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 21/30] parisc/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 22/30] openrisc/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 23/30] um/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 24/30] microblaze/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 25/30] nios2/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 26/30] hexagon/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 27/30] nds32/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 28/30] ia64/mm: " Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 29/30] mm/mmap: Drop generic vm_get_page_prot() Anshuman Khandual
2022-02-28 10:47 ` [PATCH V3 30/30] mm/mmap: Drop ARCH_HAS_VM_GET_PAGE_PROT Anshuman Khandual
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=9caa90f5-c10d-75dd-b403-1388b7a3d296@arm.com \
--to=anshuman.khandual@arm.com \
--cc=akpm@linux-foundation.org \
--cc=arnd@arndb.de \
--cc=christophe.leroy@csgroup.eu \
--cc=geert@linux-m68k.org \
--cc=hch@infradead.org \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-csky@vger.kernel.org \
--cc=linux-hexagon@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-m68k@lists.linux-m68k.org \
--cc=linux-mips@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-parisc@vger.kernel.org \
--cc=linux-riscv@lists.infradead.org \
--cc=linux-s390@vger.kernel.org \
--cc=linux-sh@vger.kernel.org \
--cc=linux-snps-arc@lists.infradead.org \
--cc=linux-um@lists.infradead.org \
--cc=linux-xtensa@linux-xtensa.org \
--cc=linux@armlinux.org.uk \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=openrisc@lists.librecores.org \
--cc=sparclinux@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).