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?
