Re: mainline build failure due to fc02735b14ff ("KVM: VMX: Prevent guest RSB poisoning attacks with eIBRS")

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 13, 2022 at 01:35:22PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 13, 2022 at 12:28:38PM +0100, Sudip Mukherjee (Codethink) wrote:
> > Hi All,
> > 
> > The latest mainline kernel branch fails to build for x86_64 allmodconfig
> > with clang and the error is:
> > 
> > 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;
> > 
> > 
> > git bisect pointed to fc02735b14ff ("KVM: VMX: Prevent guest RSB poisoning attacks with eIBRS")
> > 
> > I will be happy to test any patch or provide any extra log if needed.
> 
> I suspect something like this will do.
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index bb05ed4f46bd..6398d39e66b0 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -280,7 +280,7 @@ 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);
>  

I hate headers...

---
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index bb05ed4f46bd..cbc3b8348939 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -280,7 +280,6 @@ 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;
 extern void write_spec_ctrl_current(u64 val, bool force);
 extern u64 spec_ctrl_current(void);
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index be7c19374fdd..b64512978534 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6831,6 +6831,8 @@ void noinstr vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
 	}
 }
 
+DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
+
 void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
 					unsigned int flags)
 {