linux-kernel - Re: [PATCH] KVM: SVM: Fix build error when using -Werror=unused-but-set-variable

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Message-ID: <ZRsTd9FEIvgERrte@google.com>
Date:   Mon, 2 Oct 2023 12:01:11 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] KVM: SVM: Fix build error when using -Werror=unused-but-set-variable

On Mon, Oct 02, 2023, Tom Lendacky wrote:
> Commit 916e3e5f26ab ("KVM: SVM: Do not use user return MSR support for
> virtualized TSC_AUX") introduced a local variable used for the rdmsr()
> function for the high 32-bits of the MSR value. This variable is not used
> after being set and triggers a warning or error, when treating warnings
> as errors, when the unused-but-set-variable flag is set. Mark this
> variable as __maybe_unused to fix this.
> 
> Fixes: 916e3e5f26ab ("KVM: SVM: Do not use user return MSR support for virtualized TSC_AUX")
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/kvm/svm/svm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 9507df93f410..4c917c74a4d3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -691,7 +691,7 @@ static int svm_hardware_enable(void)
>  	 */
>  	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX)) {
>  		struct sev_es_save_area *hostsa;
> -		u32 msr_hi;
> +		u32 __maybe_unused msr_hi;

Argh, the abomination that is rdmsrl() strikes again :-/  We really should be able
to do:

		hostsa->tsc_aux = (u32)rdmsrl(MSR_TSC_AUX);

I don't see a better quick fix than __maybe_unused though.

Reviewed-by: Sean Christopherson <seanjc@...gle.com>