Re: [PATCH v2] KVM: MIPS: Fix maybe-uninitialized build failure

[James Hogan <james.hogan@imgtec.com>]

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On Tue, Jun 20, 2017 at 10:57:51AM +0100, James Cowgill wrote:
> This commit fixes a "maybe-uninitialized" build failure in
> arch/mips/kvm/tlb.c when KVM, DYNAMIC_DEBUG and JUMP_LABEL are all
> enabled. The failure is:
> 
> In file included from ./include/linux/printk.h:329:0,
>                  from ./include/linux/kernel.h:13,
>                  from ./include/asm-generic/bug.h:15,
>                  from ./arch/mips/include/asm/bug.h:41,
>                  from ./include/linux/bug.h:4,
>                  from ./include/linux/thread_info.h:11,
>                  from ./include/asm-generic/current.h:4,
>                  from ./arch/mips/include/generated/asm/current.h:1,
>                  from ./include/linux/sched.h:11,
>                  from arch/mips/kvm/tlb.c:13:
> arch/mips/kvm/tlb.c: In function ‘kvm_mips_host_tlb_inv’:
> ./include/linux/dynamic_debug.h:126:3: error: ‘idx_kernel’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    __dynamic_pr_debug(&descriptor, pr_fmt(fmt), \
>    ^~~~~~~~~~~~~~~~~~
> arch/mips/kvm/tlb.c:169:16: note: ‘idx_kernel’ was declared here
>   int idx_user, idx_kernel;
>                 ^~~~~~~~~~
> 
> There is a similar error relating to "idx_user". Both errors were
> observed with GCC 6.
> 
> As far as I can tell, it is impossible for either idx_user or idx_kernel
> to be uninitialized when they are later read in the calls to kvm_debug,
> but to satisfy the compiler, add zero initializers to both variables.
> 
> Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
> Fixes: 57e3869cfaae ("KVM: MIPS/TLB: Generalise host TLB invalidate to kernel ASID")
> Cc: <stable@vger.kernel.org> # 4.11+

Acked-by: James Hogan <james.hogan@imgtec.com>

Paolo / Radim: is it okay for one of you to pick this one up for 4.12?

Thanks
James

> ---
>  arch/mips/kvm/tlb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/kvm/tlb.c b/arch/mips/kvm/tlb.c
> index 7c6336dd2638..7cd92166a0b9 100644
> --- a/arch/mips/kvm/tlb.c
> +++ b/arch/mips/kvm/tlb.c
> @@ -166,7 +166,11 @@ static int _kvm_mips_host_tlb_inv(unsigned long entryhi)
>  int kvm_mips_host_tlb_inv(struct kvm_vcpu *vcpu, unsigned long va,
>  			  bool user, bool kernel)
>  {
> -	int idx_user, idx_kernel;
> +	/*
> +	 * Initialize idx_user and idx_kernel to workaround bogus
> +	 * maybe-initialized warning when using GCC 6.
> +	 */
> +	int idx_user = 0, idx_kernel = 0;
>  	unsigned long flags, old_entryhi;
>  
>  	local_irq_save(flags);
> -- 
> 2.11.0
> 

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