Ping: [PATCH v2] x86/PoD: move increment of entry count

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.01.2022 11:57, Jan Beulich wrote:
> When not holding the PoD lock across the entire region covering P2M
> update and stats update, the entry count should indicate too large a
> value in preference to a too small one, to avoid functions bailing early
> when they find the count is zero. Hence increments should happen ahead
> of P2M updates, while decrements should happen only after. Deal with the
> one place where this hasn't been the case yet.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Add comments.

Ping?

Jan

> ---
> While it might be possible to hold the PoD lock over the entire
> operation, I didn't want to chance introducing a lock order violation on
> a perhaps rarely taken code path.
> 
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -1342,19 +1342,22 @@ mark_populate_on_demand(struct domain *d
>          }
>      }
>  
> +    /*
> +     * Without holding the PoD lock across the entire operation, bump the
> +     * entry count up front assuming success of p2m_set_entry(), undoing the
> +     * bump as necessary upon failure.  Bumping only upon success would risk
> +     * code elsewhere observing entry count being zero despite there actually
> +     * still being PoD entries.
> +     */
> +    pod_lock(p2m);
> +    p2m->pod.entry_count += (1UL << order) - pod_count;
> +    pod_unlock(p2m);
> +
>      /* Now, actually do the two-way mapping */
>      rc = p2m_set_entry(p2m, gfn, INVALID_MFN, order,
>                         p2m_populate_on_demand, p2m->default_access);
>      if ( rc == 0 )
> -    {
> -        pod_lock(p2m);
> -        p2m->pod.entry_count += 1UL << order;
> -        p2m->pod.entry_count -= pod_count;
> -        BUG_ON(p2m->pod.entry_count < 0);
> -        pod_unlock(p2m);
> -
>          ioreq_request_mapcache_invalidate(d);
> -    }
>      else if ( order )
>      {
>          /*
> @@ -1366,6 +1369,14 @@ mark_populate_on_demand(struct domain *d
>                 d, gfn_l, order, rc);
>          domain_crash(d);
>      }
> +    else if ( !pod_count )
> +    {
> +        /* Undo earlier increment; see comment above. */
> +        pod_lock(p2m);
> +        BUG_ON(!p2m->pod.entry_count);
> +        --p2m->pod.entry_count;
> +        pod_unlock(p2m);
> +    }
>  
>  out:
>      gfn_unlock(p2m, gfn, order);
> 
>