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

From: Nathan Chancellor <nathan@kernel.org>
To: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	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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	Heiko Carstens <hca@linux.ibm.com>,
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	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: Sat, 26 Sep 2026 13:39:06 +0100	[thread overview]
Message-ID: <20260926123906.GB1486962@ax162> (raw)
In-Reply-To: <9d1c338b-368f-4ea2-a12e-c28d49adff5e@kernel.org>

On Sat, Sep 26, 2026 at 12:55:34PM +0200, Christophe Leroy (CS GROUP) wrote:
> Le 26/09/2026 à 12:02, Jason A. Donenfeld a écrit :
> > Apparently not on RISCV, which is what prompted this patch in the first
> > place. So I suspect Nathan's v2 here is the way to go.
> 
> 
> Don't you have -fno-builtin on RISCV like we have on powerpc ?
> 
> Or is it clang that is missing this option ?

Yes, clang has -fno-builtin but as I mention in the commit message, that
may not always stop the compiler from generating calls to memset(),
which it is allowed to do even with -ffreestanding (which implies
-fno-builtin) per GCC's documentation. -fno-builtin with GCC < 14 just
happens to avoid generating memset() for memset_inline() in all of my
tests, hence why I went with this approach as suggested by Jason.

-- 
Cheers,
Nathan

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