Re: [PATCH 10/14] KVM: x86: Make Hyper-V emulation optional

[Sean Christopherson]

On Wed, Oct 25, 2023, Vitaly Kuznetsov wrote:
> Hyper-V emulation in KVM is a fairly big chunk and in some cases it may be
> desirable to not compile it in to reduce module sizes as well as the attack
> surface. Introduce CONFIG_KVM_HYPERV option to make it possible.
> 
> Note, there's room for further nVMX/nSVM code optimizations when
> !CONFIG_KVM_HYPERV, this will be done in follow-up patches.
> 
> Reorganize Makefile a bit so all CONFIG_HYPERV and CONFIG_KVM_HYPERV files
> are grouped together.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---

...

> diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
> index 8ea872401cd6..b97b875ad75f 100644
> --- a/arch/x86/kvm/Makefile
> +++ b/arch/x86/kvm/Makefile
> @@ -11,32 +11,33 @@ include $(srctree)/virt/kvm/Makefile.kvm
>  
>  kvm-y			+= x86.o emulate.o i8259.o irq.o lapic.o \
>  			   i8254.o ioapic.o irq_comm.o cpuid.o pmu.o mtrr.o \
> -			   hyperv.o debugfs.o mmu/mmu.o mmu/page_track.o \
> +			   debugfs.o mmu/mmu.o mmu/page_track.o \
>  			   mmu/spte.o
>  
> -ifdef CONFIG_HYPERV
> -kvm-y			+= kvm_onhyperv.o
> -endif
> -
>  kvm-$(CONFIG_X86_64) += mmu/tdp_iter.o mmu/tdp_mmu.o
> +kvm-$(CONFIG_KVM_HYPERV) += hyperv.o
>  kvm-$(CONFIG_KVM_XEN)	+= xen.o
>  kvm-$(CONFIG_KVM_SMM)	+= smm.o
>  
>  kvm-intel-y		+= vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
> -			   vmx/hyperv.o vmx/hyperv_evmcs.o vmx/nested.o vmx/posted_intr.o
> -kvm-intel-$(CONFIG_X86_SGX_KVM)	+= vmx/sgx.o
> +			   vmx/nested.o vmx/posted_intr.o
>  
> -ifdef CONFIG_HYPERV
> -kvm-intel-y		+= vmx/vmx_onhyperv.o
> -endif
> +kvm-intel-$(CONFIG_X86_SGX_KVM)	+= vmx/sgx.o
>  
>  kvm-amd-y		+= svm/svm.o svm/vmenter.o svm/pmu.o svm/nested.o svm/avic.o \
> -			   svm/sev.o svm/hyperv.o
> +			   svm/sev.o
>  
>  ifdef CONFIG_HYPERV
> +kvm-y			+= kvm_onhyperv.o
> +kvm-intel-y		+= vmx/vmx_onhyperv.o vmx/hyperv_evmcs.o
>  kvm-amd-y		+= svm/svm_onhyperv.o
>  endif
>  
> +ifdef CONFIG_KVM_HYPERV
> +kvm-intel-y		+= vmx/hyperv.o vmx/hyperv_evmcs.o
> +kvm-amd-y		+= svm/hyperv.o
> +endif

My strong preference is to avoid the unnecessary ifdef and instead do:

kvm-intel-y             += vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
                           vmx/nested.o vmx/posted_intr.o

kvm-intel-$(CONFIG_X86_SGX_KVM) += vmx/sgx.o
kvm-intel-$(CONFIG_KVM_HYPERV)  += vmx/hyperv.o vmx/hyperv_evmcs.o

kvm-amd-y               += svm/svm.o svm/vmenter.o svm/pmu.o svm/nested.o svm/avic.o \
                           svm/sev.o
kvm-amd-$(CONFIG_KVM_HYPERV)    += svm/hyperv.o


CONFIG_HYPERV needs an ifdef because it can be 'y' or 'm', but otherwise ifdefs
just tend to be noisier.

>  static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
>  {
> @@ -3552,11 +3563,13 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>  	if (!nested_vmx_check_permission(vcpu))
>  		return 1;
>  
> +#ifdef CONFIG_KVM_HYPERV
>  	evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch);
>  	if (evmptrld_status == EVMPTRLD_ERROR) {
>  		kvm_queue_exception(vcpu, UD_VECTOR);
>  		return 1;
>  	}
> +#endif
>  
>  	kvm_pmu_trigger_event(vcpu, PERF_COUNT_HW_BRANCH_INSTRUCTIONS);

This fails to build with CONFIG_KVM_HYPERV=n && CONFIG_KVM_WERROR=y:

arch/x86/kvm/vmx/nested.c:3577:9: error: variable 'evmptrld_status' is uninitialized when used here [-Werror,-Wuninitialized]
        if (CC(evmptrld_status == EVMPTRLD_VMFAIL))
               ^~~~~~~~~~~~~~~

Sadly, simply wrapping with an #ifdef also fails because then evmptrld_status
becomes unused.  I'd really prefer to avoid having to tag it __maybe_unused, and
adding more #ifdef would also be ugly.  Any ideas?