linux-kernel - Re: [PATCH] KVM: x86: VMX: fix building without CONFIG

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Message-ID: <87d0xjwwhx.fsf@vitty.brq.redhat.com>
Date:   Fri, 25 May 2018 18:06:18 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: VMX: fix building without CONFIG_HYPERV

Arnd Bergmann <arnd@...db.de> writes:

> The global ms_hyperv variable is part of the hyperv support, so
> we get a link error from accessing it in kernels that have this
> turned off:
>
> arch/x86/kvm/vmx.o: In function `alloc_loaded_vmcs':
> vmx.c:(.text+0x1654a): undefined reference to `ms_hyperv'
> vmx.c:(.text+0x1657a): undefined reference to `ms_hyperv'
>
> This changes the condition to first check the compile-time
> configuration symbol to avoid the link error.
>
> Fixes: ceef7d10dfb6 ("KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap support")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Thanks,

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

(I don't really like this IS_ENABLED(CONFIG_HYPERV) spreading across KVM
but keeping stuff like this ms_hyperv structure defined when
!IS_ENABLED(CONFIG_HYPERV) is also ugly. It may make sense to define
helpers similar to kvm_para_has_feature() and define them to 'false'
when Hyper-V support code is compiled out. I'll look into this option
later, this patch should be taken to fix the immediate issue).


> ---
>  arch/x86/kvm/vmx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index ea098131dcce..e6d6ccab43c3 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4232,7 +4232,8 @@ static int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>  			goto out_vmcs;
>  		memset(loaded_vmcs->msr_bitmap, 0xff, PAGE_SIZE);
>
> -		if (static_branch_unlikely(&enable_evmcs) &&
> +		if (IS_ENABLED(CONFIG_HYPERV) &&
> +		    static_branch_unlikely(&enable_evmcs) &&
>  		    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
>  			struct hv_enlightened_vmcs *evmcs =
>  				(struct hv_enlightened_vmcs *)loaded_vmcs->vmcs;

-- 
  Vitaly