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

From: Nathan Chancellor <nathan@kernel.org>
To: David Laight <david.laight.linux@gmail.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	Andreas Schwab <schwab@linux-m68k.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Nick Desaulniers <ndesaulniers@google.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>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
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	Heiko Carstens <hca@linux.ibm.com>,
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	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,
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	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: Tue, 29 Sep 2026 18:59:39 +0200	[thread overview]
Message-ID: <20260929165939.GA1890629@ax162> (raw)
In-Reply-To: <20260927080225.37534e7c@pumpkin>

On Sun, Sep 27, 2026 at 08:02:25AM +0100, David Laight wrote:
> On Sat, 26 Sep 2026 14:32:49 +0200
> "Jason A. Donenfeld" <Jason@zx2c4.com> wrote:
> > Do we even need to return a value at all? Might as well just make the
> > function two lines:
> > 
> > +       for (char *d = dst; size--;)
> > +               *d++ = value;
> 
> Wouldn't it be better to add a barrier() or similar in there to
> stop the compiler playing unwanted games>

Wouldn't that just result in the same code generation issue that Jason
pointed out on my v1?

  https://lore.kernel.org/aquyVkdYceiWdckX@zx2c4.com/

Maybe that doesn't matter because modern compilers have better options?

-- 
Cheers,
Nathan

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