Re: [RFC PATCH v2 15/26] KVM: VMX: Convert vcpu_vmx.exit_reason to a union

Sean Christopherson
2021-01-21T16:35:12Z

On Thu, Jan 21, 2021, Jarkko Sakkinen wrote:
> On Wed, Jan 20, 2021 at 08:39:26AM -0800, Sean Christopherson wrote:
> > On Wed, Jan 20, 2021, Jarkko Sakkinen wrote:
> > > On Mon, Jan 18, 2021 at 04:28:26PM +1300, Kai Huang wrote:
> > > > ---
> > > > arch/x86/kvm/vmx/nested.c | 42 +++++++++++++++---------
> > > > arch/x86/kvm/vmx/vmx.c | 68 ++++++++++++++++++++-------------------
> > > > arch/x86/kvm/vmx/vmx.h | 25 +++++++++++++-
> > > > 3 files changed, 86 insertions(+), 49 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > > index 0fbb46990dfc..f112c2482887 100644
> > > > --- a/arch/x86/kvm/vmx/nested.c
> > > > +++ b/arch/x86/kvm/vmx/nested.c
> > > > @@ -3311,7 +3311,11 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> > > > struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> > > > enum vm_entry_failure_code entry_failure_code;
> > > > bool evaluate_pending_interrupts;
> > > > - u32 exit_reason, failed_index;
> > > > + u32 failed_index;
> > > > + union vmx_exit_reason exit_reason = {
> > > > + .basic = -1,
> > > > + .failed_vmentry = 1,
> > > > + };
> > >
> > > Instead, put this declaration to the correct place, following the
> > > reverse christmas tree ordering:
> > >
> > > union vmx_exit_reason exit_reason = {};
> > >
> > > And after declarations:
> > >
> > > exit_reason.basic = -1;
> > > exit_reason.failed_vmentry = 1;
> > >
> > > More pleasing for the eye.
> >
> > I disagree (obviously, since I wrote the patch). Initializing the fields to
> > their respective values is a critical, but subtle, aspect of this code. Making
> > the code stand out via explicit initialization is a good thing, and we really
> > don't want any possibility of code touching exit_reason before it is initialized.
>
> The struct does get initialized to zero, and fields get initialized
> to their respective values. What is the critical aspect that is gone?
> I'm not following.

Setting exit_reason.failed_vmentry from time zero. This code should never
synthesize a nested VM-Exit with failed_vmentry=0. There have been bugs around
the exit_reason in this code in the past, I strongly prefer to not have any
window where exit_reason has the "wrong" value.