linux-kernel - Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

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Date:   Fri, 12 Jul 2019 13:54:30 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

On Fri, Jul 12, 2019 at 03:32:43PM +0200, Arnd Bergmann wrote:
> clang points out that running a 64-bit guest on a 32-bit host
> would lead to uninitialized variables:
> 
> 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]
> 
> Since that combination is not supported anyway, change the condition
> to tell the compiler how the code is actually executed.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v2: make the change inside of is_64_bit_mode().
> ---
>  arch/x86/kvm/hyperv.c | 6 ++----
>  arch/x86/kvm/x86.h    | 4 ++++
>  2 files changed, 6 insertions(+), 4 deletions(-)

Reviewed-by: Roman Kagan <rkagan@...tuozzo.com>

However I still think the log message could state it more explicitly
that it was the compiler's fault, and the patch is a workaround for it.

Otherwise later on someone may decide to restore the similarity of
is_64_bit_mode() to other inlines in this file, and will be extremely
unlikely to test clang 32-bit build...

Roman.