On Wed, Aug 13, 2025 at 10:31:27PM -0700, Vishal Annapurve wrote:
> 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.
Ack.
> > + 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.
This step is to allocate pamt_pages for the guest 2MB page that needs splitting.
The above tdx_pamt_get() is for the EPT page to be added.
I'll add comments or update the param names for better clarity.
Regarding the absence of return value check for the tdx_alloc_pamt_pages(), I
think it's because the tdx_alloc_pamt_page_split() retrieves pages from the
pamt_page_cache via kvm_mmu_memory_cache_alloc(), which is guaranteed to succeed
(otherwise, there's a BUG_ON() in kvm_mmu_memory_cache_alloc()).
> >
> > 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?
This hpa is for guest 2MB memory range. There shouldn't have any increased
pamt_refcount for this range before a successful demote.
So, atomic_set() to PTRS_PER_PMD looks correct, though atomic_add() seems even
safer.
