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

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	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 12:48:10 +0200	[thread overview]
Message-ID: <ar46ap4jzLWyP92C@zx2c4.com> (raw)
In-Reply-To: <20261001102059.GA4176271@ax162>

On Thu, Oct 01, 2026 at 12:20:59PM +0200, Nathan Chancellor wrote:
> control. The change that introduced -max-store-memset only did it to
> "allow fine-tuning of the inlining threshold for performance analysis
> and optimization". If they decide to remove it for whatever reason,
> we're back to square one.

I suppose all the more reason to get -finline-stringops=memset added to
clang. Then the dual-default thing you came up with below will naturally
start choosing the first option when it becomes available.

> I know something like below would be uglier due to the ifdef but it
> would avoid changing anything for GCC while clearing up the issue at
> hand for clang in a guaranteed stable and succinct manner.

But then we're back to the byte-by-byte codegen that Christophe pointed
out.

> If that is not acceptable, something like the following does appear to
> work for me.

Okay, great, let's do that.

Does this commit seem okay with you? I used the diff you sent below and
adjusted the commit message: https://git.zx2c4.com/linux-rng/commit/?id=56ff95ee85715047eb5b5220243778af657778c8

Jason