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

From: Nathan Chancellor <nathan@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
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>,
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	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
Date: Thu, 1 Oct 2026 13:03:10 +0200	[thread overview]
Message-ID: <20261001110310.GA138012@ax162> (raw)
In-Reply-To: <ar46ap4jzLWyP92C@zx2c4.com>

On Thu, Oct 01, 2026 at 12:48:10PM +0200, Jason A. Donenfeld wrote:
> 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.

Fair point, I can file an issue with LLVM upstream.

> > 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.

I thought that was only because the fallback memset_inline() from v2 was
doing a byte-by-byte initialization? With my suggested diff, nothing
should change for GCC, as it does not have __builtin_memset_inline(), so
the "Before the patch" code generation that Christophe showed should
still be present.

> > 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

Yeah, that seems fine to me, thanks for taking care of it!

-- 
Cheers,
Nathan