linux-kernel - Re: [RFC PATCH v5 05/45] KVM: TDX: Drop kvm_x86_ops.link_external_spt(), use .set_external

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Message-ID: <aYJU8Som706YkIEO@google.com>
Date: Tue, 3 Feb 2026 20:05:05 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Kiryl Shutsemau <kas@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-coco@...ts.linux.dev, kvm@...r.kernel.org, 
	Kai Huang <kai.huang@...el.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>, 
	Sagi Shahar <sagis@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [RFC PATCH v5 05/45] KVM: TDX: Drop kvm_x86_ops.link_external_spt(),
 use .set_external_spte() for all

On Tue, Feb 03, 2026, Yan Zhao wrote:
> On Wed, Jan 28, 2026 at 05:14:37PM -0800, Sean Christopherson wrote:
> >  static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t sptep,
> >  						 gfn_t gfn, u64 *old_spte,
> >  						 u64 new_spte, int level)
> >  {
> > -	bool was_present = is_shadow_present_pte(*old_spte);
> > -	bool is_present = is_shadow_present_pte(new_spte);
> > -	bool is_leaf = is_present && is_last_spte(new_spte, level);
> > -	int ret = 0;
> > -
> > -	KVM_BUG_ON(was_present, kvm);
> > +	int ret;
> >  
> >  	lockdep_assert_held(&kvm->mmu_lock);
> > +
> > +	if (KVM_BUG_ON(is_shadow_present_pte(*old_spte), kvm))
> > +		return -EIO;
> Why not move this check of is_shadow_present_pte() to tdx_sept_set_private_spte()
> as well? 

The series gets there eventually, but as of this commit, @old_spte isn't plumbed
into tdx_sept_set_private_spte().

> Or also check !is_shadow_present_pte(new_spte) in TDP MMU?

Not sure I understand this suggestion.
   	
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 5688c77616e3..30494f9ceb31 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -1664,18 +1664,58 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
> >  	return 0;
> >  }
> >  
> > +static struct page *tdx_spte_to_external_spt(struct kvm *kvm, gfn_t gfn,
> > +					     u64 new_spte, enum pg_level level)
> > +{
> > +	struct kvm_mmu_page *sp = spte_to_child_sp(new_spte);
> > +
> > +	if (KVM_BUG_ON(!sp->external_spt, kvm) ||
> > +	    KVM_BUG_ON(sp->role.level + 1 != level, kvm) ||
> > +	    KVM_BUG_ON(sp->gfn != gfn, kvm))
> > +		return NULL;
> Could we remove the KVM_BUG_ON()s, and ...
> 
> > +	return virt_to_page(sp->external_spt);
> > +}
> > +
> > +static int tdx_sept_link_private_spt(struct kvm *kvm, gfn_t gfn,
> > +				     enum pg_level level, u64 mirror_spte)
> > +{
> > +	gpa_t gpa = gfn_to_gpa(gfn);
> > +	u64 err, entry, level_state;
> > +	struct page *external_spt;
> > +
> > +	external_spt = tdx_spte_to_external_spt(kvm, gfn, mirror_spte, level);
> > +	if (!external_spt)
> add a KVM_BUG_ON() here?
> It could save KVM_BUG_ON()s and have KVM_BUG_ON() match -EIO :)

We could, but I don't want to, because if we're going to bother with sanity checks,
I want the resulting WARNs to be precise.  I.e. I want the WARN to capture *why*
tdx_spte_to_external_spt() failed, to make debug/triage easier.

> And as Rick also mentioned, better to remove external in external_spt, e.g.
> something like pt_page.

Yeah, maybe sept_spt?

> And mirror_spte --> new_spte?

Hmm, ya, I made that change later, but it can probably be shifted here.

> > -	WARN_ON_ONCE(!is_shadow_present_pte(mirror_spte) ||
> > -		     (mirror_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
> > +	WARN_ON_ONCE((mirror_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
> Also check this for tdx_sept_link_private_spt()?

Eh, we could, but I don't think it's necessary.  make_nonleaf_spte() is hardcoded
to set full permissions (and I don't see that changing any time soon), whereas
leaf SPTE protections are much more dynamic.