Re: [PATCH v5 4/6] x86: extend do_mmu_update() to support returning the old PTE value

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.11.2026 11:00, Kevin Lampis wrote:
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -4123,6 +4123,7 @@ long do_mmu_update(
>          case MMU_NORMAL_PT_UPDATE:
>          case MMU_PT_UPDATE_PRESERVE_AD:
>          case MMU_PT_UPDATE_NO_TRANSLATE:
> +        case MMU_PT_UPDATE_SWAP:
>          {
>              p2m_type_t p2mt;
>  
> @@ -4184,37 +4185,79 @@ long do_mmu_update(
>                       update_flags = PTE_UPDATE_PRESERVE_AD;
>                  else if ( cmd == MMU_PT_UPDATE_NO_TRANSLATE )
>                       update_flags = PTE_UPDATE_NO_TRANSLATE;
> +                else if ( cmd == MMU_PT_UPDATE_SWAP )
> +                     update_flags = PTE_UPDATE_SWAP;
>  
>                  switch ( page->u.inuse.type_info & PGT_type_mask )
>                  {
>                  case PGT_l1_page_table:
> +                {
> +                    l1_pgentry_t ol1e = {};
>                      rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
> -                                      update_flags, v, pg_owner, NULL);
> -                    break;
> +                                      update_flags, v, pg_owner, &ol1e);
>  
> +                    if ( !rc && cmd == MMU_PT_UPDATE_SWAP )
> +                    {
> +                        req.val = ol1e.l1;

If you didn't open-code l1e_get_intpte() here, it would become more obvious
that the INTPTE -> L<n>E done in mod_l<n>_entry() is simply being reversed
here. If ol1e was of type intpte_t, doing the update in update_intpte() (and
not UPDATE_ENTRY()) would become feasible. Since intpte_t is a mere alias of
uint64_t, passing &req.val into mod_l<n>_entry() directly would then also
become possible. Less redundancy would be the result.

> +                        if ( unlikely(copy_field_to_guest(ureqs, &req, val)) 
> )
> +                            rc = -EFAULT;
> +                    }
> +                    break;
> +                }
>                  case PGT_l2_page_table:
> +                {

If changes next to these case labels would remain, please also add the so
far missing blank lines between non-fall-through case blocks.

Jan