Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

From: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, loongarch@lists.linux.dev,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	linux-s390@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
Date: Thu, 1 Oct 2026 06:41:20 +0200	[thread overview]
Message-ID: <ede392d7-3104-4a0f-9fa1-99b22552dcd6@kernel.org> (raw)
In-Reply-To: <CAHmME9pA+AntJ6hWoFYpTVjh-NUSd0ZGXp5XiUuhQcq3HjQT3g@mail.gmail.com>

Hi,

Le 30/09/2026 à 17:16, Jason A. Donenfeld a écrit :
> On Wed, Sep 30, 2026 at 5:13 PM Nathan Chancellor <nathan@kernel.org> wrote:
>>
>> On Wed, Sep 30, 2026 at 04:44:29PM +0200, Jason A. Donenfeld wrote:
>>> Nathan, would this be okay with you?
>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3Dd216701724b7d8209ff42150658ad5c712bdb503&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324080837%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=BA7Fyr0%2FynggvBnR%2FpiCvNCQHtNBagqvSb0i11AvoLU%3D&reserved=0
>>
>> Can you stick
>>
>>    Cc: stable@vger.kernel.org # v6.12+
>>    Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F2183&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324110679%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6C8%2FAEQoh%2B8pfgfYQkL7qf5Ue%2F5sAGtfqRptzsSG0CM%3D&reserved=0
>>
>> on that? Otherwise, looks good to me, that's basically what I had for my
>> v3 locally.
> 
> Sure, done. Also removed the now-unused array_size.h include.

I'm still very sceptic with this patch. You are degrading the behaviour 
with GCC for a problem with CLANG. Why ?

Before the patch, with both GCC 13 and GCC 16 on powerpc32 I get a 
pretty standard optimised loop that clears words 4 by 4 (with auto 
increment of pointer) which is the most optimal on powerpc:

  3f0:	39 00 00 0c 	li      r8,12
  3f4:	35 08 ff fc 	addic.  r8,r8,-4
  3f8:	91 49 00 04 	stw     r10,4(r9)
  3fc:	91 49 00 08 	stw     r10,8(r9)
  400:	91 49 00 0c 	stw     r10,12(r9)
  404:	95 49 00 10 	stwu    r10,16(r9)
  408:	40 82 ff ec 	bne     3f4 <__c_kernel_getrandom+0x3f4>

With the patch,

With GCC 13 I get a very suboptimal loop copying bytes one by one

  3d8:	39 40 00 34 	li      r10,52
...
  3e4:	39 20 00 00 	li      r9,0
  3e8:	7d 49 03 a6 	mtctr   r10
  3ec:	9d 3e 00 01 	stbu    r9,1(r30)
  3f0:	42 00 ff fc 	bdnz    3ec <__c_kernel_getrandom+0x3ec>

With GCC 16 I get something a bit better but not as good as before, it 
is a loop clearing words only one by one and incrementing pointer with 
an additional insn instead of using auto-increment instruction stwu.

  3e0:	39 40 00 0d 	li      r10,13
...
  3f0:	7d 49 03 a6 	mtctr   r10
  3f4:	91 3f 00 00 	stw     r9,0(r31)
  3f8:	3b ff 00 04 	addi    r31,r31,4
  3fc:	42 00 ff f8 	bdnz    3f4 <__c_kernel_getrandom+0x3f4>

Please restrict the patch to clang builds.

Christophe

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