Re: [RFC PATCH v2 22/23] KVM: TDX: Handle Dynamic PAMT on page split

[Vishal Annapurve]

On Thu, Aug 7, 2025 at 2:46 AM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> +static struct page *tdx_alloc_pamt_page_split(void *data)
> +{
> +       struct kvm *kvm = data;
> +       void *p;
> +
> +       p = kvm_mmu_memory_cache_alloc(&kvm->arch.pamt_page_cache);
> +       return virt_to_page(p);
> +}
> +
>  static int tdx_spte_demote_private_spte(struct kvm *kvm, gfn_t gfn,
> -                                       enum pg_level level, struct page *page)
> +                                       enum pg_level level, struct page *page,
> +                                       kvm_pfn_t pfn_for_gfn)
>  {
>         int tdx_level = pg_level_to_tdx_sept_level(level);
> +       hpa_t hpa = pfn_to_hpa(pfn_for_gfn);
>         struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
>         gpa_t gpa = gfn_to_gpa(gfn);
>         u64 err, entry, level_state;
> +       LIST_HEAD(pamt_pages);
> +
> +       tdx_pamt_get(page, PG_LEVEL_4K, tdx_alloc_pamt_page_split, kvm);

This invocation needs a return value check.

> +       tdx_alloc_pamt_pages(&pamt_pages, tdx_alloc_pamt_page_split, kvm);

IIUC tdx_pamt_get() will result in pamt_pages allocation above, so
this step is not needed.

>
>         err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
> -                                 NULL, &entry, &level_state);
> +                                 &pamt_pages, &entry, &level_state);
>
>         if (unlikely(tdx_operand_busy(err))) {
>                 tdx_no_vcpus_enter_start(kvm);
>                 err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
> -                                         NULL, &entry, &level_state);
> +                                         &pamt_pages, &entry, &level_state);
>                 tdx_no_vcpus_enter_stop(kvm);
>         }
>
>         if (KVM_BUG_ON(err, kvm)) {
> +               tdx_free_pamt_pages(&pamt_pages);

If tdx_alloc_pamt_pages() is not needed then this can be dropped as well.

> +               tdx_pamt_put(page, PG_LEVEL_4K);
>                 pr_tdx_error_2(TDH_MEM_PAGE_DEMOTE, err, entry, level_state);
>                 return -EIO;
>         }
> +
> +       if (tdx_supports_dynamic_pamt(tdx_sysinfo))
> +               atomic_set(tdx_get_pamt_refcount(hpa), PTRS_PER_PMD);

Should this be
atomic_set(tdx_get_pamt_refcount(hpa), PTRS_PER_PMD -1 );

as tdx_pamt_get would have increased the refcount by 1 already above?