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

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Date:   Wed, 13 Jul 2022 09:27:03 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 06:22:41PM +0200, Peter Zijlstra wrote:
> 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?

Yes, I did my regular set of x86 builds with clang and they all passed.

Cheers,
Nathan