Re: [PATCH] x86/PoD: defer nested P2M flushes

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 11, 2021 at 10:17:08AM +0200, Jan Beulich wrote:
> With NPT or shadow in use, the p2m_set_entry() -> p2m_pt_set_entry() ->
> write_p2m_entry() -> p2m_flush_nestedp2m() call sequence triggers a lock
> order violation when the PoD lock is held around it. Hence such flushing
> needs to be deferred. Steal the approach from p2m_change_type_range().
> 
> Similarly for EPT I think ept_set_entry() -> ept_sync_domain() ->
> ept_sync_domain_prepare() -> p2m_flush_nestedp2m() is affected.
> 
> Reported-by: Elliott Mitchell <ehem+xen@xxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -24,6 +24,7 @@
>  #include <xen/mm.h>
>  #include <xen/sched.h>
>  #include <xen/trace.h>
> +#include <asm/hvm/nestedhvm.h>
>  #include <asm/page.h>
>  #include <asm/paging.h>
>  #include <asm/p2m.h>
> @@ -494,6 +495,13 @@ p2m_pod_offline_or_broken_replace(struct
>  static int
>  p2m_pod_zero_check_superpage(struct p2m_domain *p2m, gfn_t gfn);
>  
> +static void pod_unlock_and_flush(struct p2m_domain *p2m)
> +{
> +    pod_unlock(p2m);
> +    p2m->defer_nested_flush = false;
> +    if ( nestedhvm_enabled(p2m->domain) )
> +        p2m_flush_nestedp2m(p2m->domain);
> +}
>  
>  /*
>   * This function is needed for two reasons:
> @@ -514,6 +522,7 @@ p2m_pod_decrease_reservation(struct doma
>  
>      gfn_lock(p2m, gfn, order);
>      pod_lock(p2m);
> +    p2m->defer_nested_flush = true;
>  
>      /*
>       * If we don't have any outstanding PoD entries, let things take their
> @@ -665,7 +674,7 @@ out_entry_check:
>      }
>  
>  out_unlock:
> -    pod_unlock(p2m);
> +    pod_unlock_and_flush(p2m);
>      gfn_unlock(p2m, gfn, order);
>      return ret;
>  }
> @@ -1144,8 +1153,10 @@ p2m_pod_demand_populate(struct p2m_domai
>       * won't start until we're done.
>       */
>      if ( unlikely(d->is_dying) )
> -        goto out_fail;
> -
> +    {
> +        pod_unlock(p2m);
> +        return false;
> +    }
>  
>      /*
>       * Because PoD does not have cache list for 1GB pages, it has to remap
> @@ -1167,6 +1178,8 @@ p2m_pod_demand_populate(struct p2m_domai
>                                p2m_populate_on_demand, p2m->default_access);
>      }
>  
> +    p2m->defer_nested_flush = true;
> +
>      /* Only reclaim if we're in actual need of more cache. */
>      if ( p2m->pod.entry_count > p2m->pod.count )
>          pod_eager_reclaim(p2m);
> @@ -1229,8 +1242,9 @@ p2m_pod_demand_populate(struct p2m_domai
>          __trace_var(TRC_MEM_POD_POPULATE, 0, sizeof(t), &t);
>      }
>  
> -    pod_unlock(p2m);
> +    pod_unlock_and_flush(p2m);
>      return true;
> +
>  out_of_memory:
>      pod_unlock(p2m);

Don't you need to set defer_nested_flush = false in the out_of_memory
label? (as you don't call pod_unlock_and_flush that would do it)

Thanks, Roger.