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

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 01/10/2026 à 12:48, Jason A. Donenfeld a écrit :
> 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3D56ff95ee85715047eb5b5220243778af657778c8&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cd89b4de22ff94b6be71208df1fa9839a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639264485055490029%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Vxf8xRCdLavtw4Dmi%2B49FY1lJDWtm%2BjKv7f8BxKTGq4%3D&reserved=0
> 

The commit message says: Similarly, GCC has -finline-stringops=memset to 
do the same [4], should this issue ever hit future version of GCC.

Why default "-finline-stringops=memset" if 
$(cc-option,-finline-stringops=memset), have we identified cases where 
build fail without that or is it just for future provision ?

As shown in my previous email, with GCC 16 on powerpc32 we get a 
slightly better code without this option.

---
diff --git a/init/Kconfig b/init/Kconfig
index 8583d9f06c52..98d8d7fa64c5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -173,6 +173,11 @@ config CC_HAS_ALLOC_TOKEN
  config CC_HAS_MULTIDIMENSIONAL_NONSTRING
  	def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) 
= { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)

+config CC_OPT_INLINE_MEMSET
+	string
+	default "-finline-stringops=memset" if 
$(cc-option,-finline-stringops=memset)
+	default "-mllvm -max-store-memset=4294967295" if $(cc-option,-mllvm 
-max-store-memset=4294967295)
+
  config LD_CAN_USE_KEEP_IN_OVERLAY
  	# ld.lld prior to 21.0.0 did not support KEEP within an overlay 
description
  	# https://github.com/llvm/llvm-project/pull/130661