Subject: Re: [PATCH v4 12/28] KVM: TDX: WARN if mirror SPTE doesn't have full RWX when creating S-EPT mapping

From: Sean Christopherson <[email-masked]>
Date: Thu, 30 Oct 2025 16:40:40 -0700

On Thu, Oct 30, 2025, Kai Huang wrote:
> On Thu, 2025-10-30 at 13:09 -0700, Sean Christopherson wrote:
> > Pass in the mirror_spte to kvm_x86_ops.set_external_spte() to provide
> > symmetry with .remove_external_spte(), and assert in TDX that the mirror
> > SPTE is shadow-present with full RWX permissions (the TDX-Module doesn't
> > allow the hypervisor to control protections).
> > 
> > Signed-off-by: Sean Christopherson <[email-masked]>
> 
> Reviewed-by: Kai Huang <[email-masked]>
> 
> [...]
> 
> >  static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
> > -				     enum pg_level level, kvm_pfn_t pfn)
> > +				     enum pg_level level, u64 mirror_spte)
> >  {
> >  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	kvm_pfn_t pfn = spte_to_pfn(mirror_spte);
> >  
> >  	/* TODO: handle large pages. */
> >  	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> >  		return -EIO;
> >  
> > +	WARN_ON_ONCE(!is_shadow_present_pte(mirror_spte) ||
> > +		     (mirror_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
> > +
> 
> Nit: 
> 
> I am a little bit confused about when to use WARN_ON_ONCE() and
> KVM_BUG_ON(). :-)

Very loosely: WARN if there's a decent chance carrying on might be fine,
KVM_BUG_ON() if there's a good chance carrying on will crash the host and/or
corrupt the guest, e.g. if KVM suspects a hardware/TDX-Module issue.