linux-kernel - Re: [PATCH 11/14] KVM: selftests: Disable "gnu-variable-sized-type-not-at-end" warning

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Date:   Mon, 12 Dec 2022 17:36:58 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nathan Chancellor <nathan@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Tom Rix <trix@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Ricardo Koller <ricarkol@...gle.com>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Raghavendra Rao Ananta <rananta@...gle.com>
Subject: Re: [PATCH 11/14] KVM: selftests: Disable "gnu-variable-sized-type-not-at-end"
 warning

On Mon, Dec 12, 2022 at 4:17 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Disable gnu-variable-sized-type-not-at-end so that tests and libraries
> can create overlays of variable sized arrays at the end of structs when
> using a fixed number of entries, e.g. to get/set a single MSR.
>
> It's possible to fudge around the warning, e.g. by defining a custom
> struct that hardcodes the number of entries, but that is a burden for
> both developers and readers of the code.
>
> lib/x86_64/processor.c:664:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>                 struct kvm_msrs header;
>                                 ^
> lib/x86_64/processor.c:772:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>                 struct kvm_msrs header;
>                                 ^
> lib/x86_64/processor.c:787:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>                 struct kvm_msrs header;
>                                 ^
> 3 warnings generated.
>
> x86_64/hyperv_tlb_flush.c:54:18: warning: field 'hv_vp_set' with variable sized type 'struct hv_vpset'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>         struct hv_vpset hv_vp_set;
>                         ^
> 1 warning generated.
>
> x86_64/xen_shinfo_test.c:137:25: warning: field 'info' with variable sized type 'struct kvm_irq_routing'
> not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>         struct kvm_irq_routing info;
>                                ^
> 1 warning generated.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  tools/testing/selftests/kvm/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 2487db21b177..9cff99a1cb2e 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -196,6 +196,7 @@ else
>  LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
>  endif
>  CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
> +       -Wno-gnu-variable-sized-type-not-at-end \

This is a clang-specific warning. This will need to be wrapped in a
cc-option check.

tools/build/Build.include seems to redefine that make macro, so be
sure to test it first.

>         -fno-builtin-memcmp -fno-builtin-memcpy -fno-builtin-memset \
>         -fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
>         -I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
> --
> 2.39.0.rc1.256.g54fd8350bd-goog
>
>


-- 
Thanks,
~Nick Desaulniers