Re: [PATCH v4 08/17] KVM: TDX: Adjust the topup count of DPAMT page pairs for splitting S-EPT

[Yan Zhao]

On Thu, Oct 22, 2026 at 11:56:47PM +0800, Edgecombe, Rick P wrote:
> On Thu, 2026-10-22 at 15:24 +0800, Yan Zhao wrote:
> > static int tdx_topup_external_pamt_cache(struct kvm *kvm, struct kvm_vcpu
> > *vcpu,
> >                                          bool with_root_spt, int min_nr_spts)
> > {
> >         struct tdx_pamt_cache *pamt_cache;
> >         int dpamt_pairs = min_nr_spts;
> > 
> >         pamt_cache = tdx_get_pamt_cache(kvm, vcpu);
> >         if (!pamt_cache)
> >                 return -EIO;
> > 
> >         /* DPAMT page pair for the root SPT has already been installed */
> >         if (with_root_spt)
> >                 dpamt_pairs -= 1;
> > 
> >         /* +1 pair for 4KB guest private page */
> >         dpamt_pairs += 1;
> > 
> >         return tdx_topup_pamt_cache(pamt_cache, dpamt_pairs);
> > }
> 
> Yes this looks much clearer than even the upstream code. Do you have any better
> ideas than "4KB guest private page" though? Because in the split case it is
> actually for 512 guest private pages. We should put an artful comment to explain
> both cases while not making it look like something that has a better solution.
> Because I'm thinking there isn't one.

What about a comment like
	/*
         * +1 pair to account for mapping a new 4KB guest page or splitting a
         * 2MB guest page into 512 4KB pages.
         */
?

> > In the fault path, mmu_topup_memory_caches() calls:
> > r = kvm_x86_call(topup_external_cache)(vcpu->kvm, vcpu, true,
> > PT64_ROOT_MAX_LEVEL);
> > 
> > In the split path, tdp_mmu_alloc_sp_for_split() calls:
> > kvm_x86_call(topup_external_cache)(kvm, kvm_get_running_vcpu(), false, 1);
>