linux-kernel - Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode is enabled

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Date:   Tue, 14 Jul 2020 06:21:21 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode
 is enabled

On Tue, Jul 14, 2020 at 02:00:04PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> 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().