linux-kernel - Re: [PATCH v2] x86/speculation: Use DECLARE_PER_CPU for x86_spec_ctrl

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Date:   Wed, 13 Jul 2022 18:22:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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 v2] x86/speculation: Use DECLARE_PER_CPU for
 x86_spec_ctrl_current

On Wed, Jul 13, 2022 at 08:24:37AM -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>
> ---
> 
> v1 -> v2: https://lore.kernel.org/20220713152222.1697913-1-nathan@kernel.org/
> 
> * Use asm/percpu.h instead of linux/percpu.h to avoid static call
>   include errors.
> 
>  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..10a3bfc1eb23 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -11,6 +11,7 @@
>  #include <asm/cpufeatures.h>
>  #include <asm/msr-index.h>
>  #include <asm/unwind_hints.h>
> +#include <asm/percpu.h>
>  
>  #define RETPOLINE_THUNK_SIZE	32
>  

When I tried this earlier today I ran into cyclic headers, you sure this
works?