linux-kernel - Re: [PATCH] x86/speculation: Use DECLARE_PER_CPU for x86_spec_ctrl

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Date:   Wed, 13 Jul 2022 08:24:01 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        stable@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] x86/speculation: Use DECLARE_PER_CPU for
 x86_spec_ctrl_current

On Wed, Jul 13, 2022 at 08:22:22AM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
>   arch/x86/kernel/cpu/bugs.c:58:21: error: section attribute is specified on redeclared variable [-Werror,-Wsection]
>   DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
>                       ^
>   arch/x86/include/asm/nospec-branch.h:283:12: note: previous declaration is here
>   extern u64 x86_spec_ctrl_current;
>              ^
>   1 error generated.
> 
> The declaration should be using DECLARE_PER_CPU instead so all
> attributes stay in sync.
> 
> Cc: stable@...r.kernel.org
> Fixes: fc02735b14ff ("KVM: VMX: Prevent guest RSB poisoning attacks with eIBRS")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>  arch/x86/include/asm/nospec-branch.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index bb05ed4f46bd..99a29c83adf8 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -6,6 +6,7 @@
>  #include <linux/static_key.h>
>  #include <linux/objtool.h>
>  #include <linux/linkage.h>
> +#include <linux/percpu.h>

Argh, forgot to amend... v2 incoming.

>  #include <asm/alternative.h>
>  #include <asm/cpufeatures.h>
> @@ -280,7 +281,7 @@ static inline void indirect_branch_prediction_barrier(void)
>  
>  /* The Intel SPEC CTRL MSR base value cache */
>  extern u64 x86_spec_ctrl_base;
> -extern u64 x86_spec_ctrl_current;
> +DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
>  extern void write_spec_ctrl_current(u64 val, bool force);
>  extern u64 spec_ctrl_current(void);
>  
> 
> base-commit: 72a8e05d4f66b5af7854df4490e3135168694b6b
> -- 
> 2.37.1
>