On Tue, Jul 14, 2020 at 02:00:04PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 95ef629228691..5f526d94c33f3 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -819,22 +819,22 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
> > if ((cr0 & X86_CR0_PG) && !(cr0 & X86_CR0_PE))
> > return 1;
> >
> > - if (cr0 & X86_CR0_PG) {
> > #ifdef CONFIG_X86_64
> > - if (!is_paging(vcpu) && (vcpu->arch.efer & EFER_LME)) {
> > - int cs_db, cs_l;
> > + if ((vcpu->arch.efer & EFER_LME) && !is_paging(vcpu) &&
> > + (cr0 & X86_CR0_PG)) {
>
> it seems we have more than one occurance of "if (vcpu->arch.efer &
> EFER_LME)" under "#ifdef CONFIG_X86_64" and we alredy have
>
> static inline int is_long_mode(struct kvm_vcpu *vcpu)
> {
> #ifdef CONFIG_X86_64
> return vcpu->arch.efer & EFER_LMA;
> #else
> return 0;
> #endif
> }
>
> so if we use this instead, the compilers will just throw away the
> non-reachable blocks when !(#ifdef CONFIG_X86_64), right?
EFER.LME vs. EFER.LMA. The kvm_set_cr0() check is specifically looking at
the case where EFER.LME=1, EFER.LMA=0, and CR0.PG is being toggled on, i.e.
long mode is being enabled. EFER_LMA won't be set until vmx_set_cr0() does
enter_lmode().
