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

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Message-ID: <00f4bddd2e7bf7ab1b1718f97bb14f2c85bbf0f3.camel@intel.com>
Date: Thu, 30 Oct 2025 23:59:35 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
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Subject: Re: [PATCH v4 12/28] KVM: TDX: WARN if mirror SPTE doesn't have full
 RWX when creating S-EPT mapping

On Thu, 2025-10-30 at 16:40 -0700, Sean Christopherson wrote:
> 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 <seanjc@...gle.com>
> > 
> > Reviewed-by: Kai Huang <kai.huang@...el.com>
> > 
> > [...]
> > 
> > >  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.

Makes sense.  Thanks.