Re: [PATCH v4 02/17] x86/virt/tdx: Add a SEAMCALL wrapper to demote a 2MB huge page

[Yan Zhao]

On Thu, Oct 15, 2026 at 02:11:51PM +0800, Yan Zhao wrote:
> > > u64 tdh_mem_page_demote_2mb(struct tdx_td *td, u64 gpa, kvm_pfn_t pfn, struct page *new_sept_pt,
> > >                             struct tdx_pamt_cache *pamt_cache, u64 *ext_err1, u64 *ext_err2)
> > > {
> > >         struct tdx_module_args args = {
> > >                 .rcx = gpa | pg_level_to_tdx_sept_level(PG_LEVEL_2M),
> > >                 .rdx = tdx_tdr_pa(td),
> > >                 .r8 = page_to_phys(new_sept_pt),
> > >         };
> > >         atomic_t *dpamt_refcount = tdx_find_dpamt_refcount(pfn);
Oh, when DPAMT is not enabled, this line makes it erroneous to access the
uninitialized dpamt_refcounts.

> > >         struct page *dpamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
> > >         *ext_err1 = 0;
> > >         *ext_err2 = 0;
> > >         u64 ret;
> > > 
> > >         if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > >                 return TDX_SW_ERROR;
> > 
> > Do we need to handle this logically? The below will return some error in any
So, we need this check and should move tdx_find_dpamt_refcount() after it.

Would the below version look good to you?

Changes vs the version in
https://lore.kernel.org/all/as9OHnM4mceEre%2F2@xxxxxxxxxxxxxxxxxxxxxxxxx:
- Added WARN_ON_ONCE() for tdx_supports_dynamic_pamt() and moved
  tdx_find_dpamt_refcount() after checking DPAMT is enabled.
- Added WARN_ON_ONCE() for DEMOTE SEAMCALL failure.
- atomic_cmpxchg_release() to atomic_set() and added
  WARN_ON_ONCE(atomic_read(dpamt_refcount)).

(I didn't add the warning for alloc_pamt_array(). Will add it if you think the
justification in
https://lore.kernel.org/all/atBupzmYqvnB0Qcu@xxxxxxxxxxxxxxxxxxxxxxxxx makes sense.


u64 tdh_mem_page_demote_2mb(struct tdx_td *td, u64 gpa, kvm_pfn_t pfn, struct page *new_sept_pt,
                            struct tdx_pamt_cache *pamt_cache, u64 *ext_err1, u64 *ext_err2)
{
        struct tdx_module_args args = {
                .rcx = gpa | pg_level_to_tdx_sept_level(PG_LEVEL_2M),
                .rdx = tdx_tdr_pa(td),
                .r8 = page_to_phys(new_sept_pt),
        };
        struct page *dpamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
        atomic_t *dpamt_refcount;
        u64 ret;

        *ext_err1 = 0;
        *ext_err2 = 0;

        if (WARN_ON_ONCE(!tdx_supports_dynamic_pamt(&tdx_sysinfo)))
                return TDX_SW_ERROR;

        dpamt_refcount = tdx_find_dpamt_refcount(pfn);

        ret = alloc_pamt_array(dpamt_pages, pamt_cache);
        if (ret)
                return TDX_SW_ERROR;

        args.r12 = page_to_phys(dpamt_pages[0]),
        args.r13 = page_to_phys(dpamt_pages[1]),

        /*
         * The dpamt_lock is used to avoid contention between the
         * TDH_MEM_PAGE_DEMOTE and TDH_PHYMEM_PAMT_{ADD/REMOVE} SEAMCALLs.
         */
        spin_lock(&dpamt_lock);

        WARN_ON_ONCE(atomic_read(dpamt_refcount));

        ret = seamcall_saved_ret(TDH_MEM_PAGE_DEMOTE, &args);

        *ext_err1 = args.rcx;
        *ext_err2 = args.rdx;

        if (WARN_ON_ONCE(ret != TDX_SUCCESS))
                goto out_free;

        atomic_set(dpamt_refcount, PTRS_PER_PMD);
        spin_unlock(&dpamt_lock);

        return TDX_SUCCESS;

out_free:
        spin_unlock(&dpamt_lock);
        reinsert_pamt_array(pamt_cache, dpamt_pages);

        return ret;
}
EXPORT_SYMBOL_FOR_KVM(tdh_mem_page_demote_2mb);