linux-kernel - Re: [PATCH] [v3] x86: kvm: avoid -Wsometimes-uninitized warning

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Date:   Fri, 12 Jul 2019 19:45:55 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] [v3] x86: kvm: avoid -Wsometimes-uninitized warning

On 12/07/19 16:13, Arnd Bergmann wrote:
> Clang notices a code path in which some variables are never
> initialized, but fails to figure out that this can never happen
> on i386 because is_64_bit_mode() always returns false.
> 
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>         if (!longmode) {
>             ^~~~~~~~~
> arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
>         trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
>                                                              ^~~~~
> arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
>         if (!longmode) {
>         ^~~~~~~~~~~~~~~
> arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
>         u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
>                         ^
>                          = 0
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> 
> Flip the condition around to avoid the conditional execution on i386.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v3: reword commit log, simplify patch again
> v2: make the change inside of is_64_bit_mode().
> ---
>  arch/x86/kvm/hyperv.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index a39e38f13029..c10a8b10b203 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1594,7 +1594,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  {
>  	u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
>  	uint16_t code, rep_idx, rep_cnt;
> -	bool fast, longmode, rep;
> +	bool fast, rep;
>  
>  	/*
>  	 * hypercall generates UD from non zero cpl and real mode
> @@ -1605,9 +1605,14 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  		return 1;
>  	}
>  
> -	longmode = is_64_bit_mode(vcpu);
> -
> -	if (!longmode) {
> +#ifdef CONFIG_X86_64
> +	if (is_64_bit_mode(vcpu)) {
> +		param = kvm_rcx_read(vcpu);
> +		ingpa = kvm_rdx_read(vcpu);
> +		outgpa = kvm_r8_read(vcpu);
> +	} else
> +#endif
> +	{
>  		param = ((u64)kvm_rdx_read(vcpu) << 32) |
>  			(kvm_rax_read(vcpu) & 0xffffffff);
>  		ingpa = ((u64)kvm_rbx_read(vcpu) << 32) |
> @@ -1615,13 +1620,6 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  		outgpa = ((u64)kvm_rdi_read(vcpu) << 32) |
>  			(kvm_rsi_read(vcpu) & 0xffffffff);
>  	}
> -#ifdef CONFIG_X86_64
> -	else {
> -		param = kvm_rcx_read(vcpu);
> -		ingpa = kvm_rdx_read(vcpu);
> -		outgpa = kvm_r8_read(vcpu);
> -	}
> -#endif
>  
>  	code = param & 0xffff;
>  	fast = !!(param & HV_HYPERCALL_FAST_BIT);
> 

Queued, thanks.

Paolo