Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable

[Qian Cai]

On Tue, 2020-09-29 at 14:26 +0200, Paolo Bonzini wrote:
> On 29/09/20 13:59, Qian Cai wrote:
> > WARN_ON_ONCE(!allow_smaller_maxphyaddr);
> > 
> > I noticed the origin patch did not have this WARN_ON_ONCE(), but the
> > mainline
> > commit b96e6506c2ea ("KVM: x86: VMX: Make smaller physical guest address
> > space
> > support user-configurable") does have it for some reasons.
> 
> Because that part of the code should not be reached.  The exception
> bitmap is set up with
> 
>         if (!vmx_need_pf_intercept(vcpu))
>                 eb &= ~(1u << PF_VECTOR);
> 
> where
> 
> static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
> {
>         if (!enable_ept)
>                 return true;
> 
>         return allow_smaller_maxphyaddr &&
> 		 cpuid_maxphyaddr(vcpu) < boot_cpu_data.x86_phys_bits;
> }
> 
> We shouldn't get here if "enable_ept && !allow_smaller_maxphyaddr",
> which implies vmx_need_pf_intercept(vcpu) == false.  So the warning is
> genuine; I've sent a patch.

Care to provide a link to the patch? Just curious.