linux-kernel - Re: [PATCH 3/3] KVM: x86: announce KVM_CAP_HYPERV_ENLIGHTENED

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Date:   Tue, 27 Aug 2019 12:52:24 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH 3/3] KVM: x86: announce KVM_CAP_HYPERV_ENLIGHTENED_VMCS
 support only when it is available

On Tue, Aug 27, 2019 at 09:54:39AM -0700, Jim Mattson wrote:
> On Tue, Aug 27, 2019 at 9:04 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >
> > It was discovered that after commit 65efa61dc0d5 ("selftests: kvm: provide
> > common function to enable eVMCS") hyperv_cpuid selftest is failing on AMD.
> > The reason is that the commit changed _vcpu_ioctl() to vcpu_ioctl() in the
> > test and this one can't fail.
> >
> > Instead of fixing the test is seems to make more sense to not announce
> > KVM_CAP_HYPERV_ENLIGHTENED_VMCS support if it is definitely missing
> > (on svm and in case kvm_intel.nested=0).
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> > ---
> >  arch/x86/kvm/x86.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index d1cd0fcff9e7..ef2e8b138300 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3106,7 +3106,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >         case KVM_CAP_HYPERV_EVENTFD:
> >         case KVM_CAP_HYPERV_TLBFLUSH:
> >         case KVM_CAP_HYPERV_SEND_IPI:
> > -       case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
> >         case KVM_CAP_HYPERV_CPUID:
> >         case KVM_CAP_PCI_SEGMENT:
> >         case KVM_CAP_DEBUGREGS:
> > @@ -3183,6 +3182,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >                 r = kvm_x86_ops->get_nested_state ?
> >                         kvm_x86_ops->get_nested_state(NULL, NULL, 0) : 0;
> >                 break;
> > +       case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
> > +               r = kvm_x86_ops->nested_enable_evmcs != NULL;
> 
> You should probably have an explicit break here, in case someone later
> adds another case below.

Yep, this will trigger a warning on compilers with -Wimplicit-fallthrough.