Re: [RFC PATCH] x86/p2m-pt: do type recalculations with p2m read lock

[Jan Beulich <jbeulich@suse.com>]

On 03.04.2023 12:14, Roger Pau Monne wrote:
> --- a/xen/arch/x86/mm/p2m-pt.c
> +++ b/xen/arch/x86/mm/p2m-pt.c
> @@ -486,9 +486,6 @@ static int cf_check do_recalc(struct p2m_domain *p2m, unsigned long gfn)
>          p2m_type_t ot, nt;
>          unsigned long mask = ~0UL << (level * PAGETABLE_ORDER);
>  
> -        if ( !valid_recalc(l1, e) )
> -            P2M_DEBUG("bogus recalc leaf at d%d:%lx:%u\n",
> -                      p2m->domain->domain_id, gfn, level);
>          ot = p2m_flags_to_type(l1e_get_flags(e));
>          nt = p2m_recalc_type_range(true, ot, p2m, gfn & mask, gfn | ~mask);
>          if ( nt != ot )

I'm afraid I neither understand why you make this change, nor why you
then leave the other use of valid_recalc() in place.

> @@ -538,9 +535,9 @@ int p2m_pt_handle_deferred_changes(uint64_t gpa)
>       */
>      ASSERT(!altp2m_active(current->domain));
>  
> -    p2m_lock(p2m);
> +    p2m_read_lock(p2m);
>      rc = do_recalc(p2m, PFN_DOWN(gpa));
> -    p2m_unlock(p2m);
> +    p2m_read_unlock(p2m);
>  
>      return rc;
>  }

How can this be safe, when do_recalc() involves p2m_next_level(), which
may install new (intermediate) page tables?

Jan