RE: [PATCH v2] x86/PoD: defer nested P2M flushes

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Tuesday, October 19, 2021 8:52 PM
> 
> 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().
> (Note that strictly speaking the change at the out_of_memory label is
> not needed, as the domain gets crashed there anyway. The change is being
> made nevertheless to avoid setting up a trap from someone meaning to
> deal with that case better than by domain_crash().)
> 
> Similarly for EPT I think ept_set_entry() -> ept_sync_domain() ->
> ept_sync_domain_prepare() -> p2m_flush_nestedp2m() is affected. Make its
> p2m_flush_nestedp2m() invocation conditional. Note that this then also
> alters behavior of p2m_change_type_range() on EPT, deferring the nested
> flushes there as well. I think this should have been that way from the
> introduction of the flag.
> 
> Reported-by: Elliott Mitchell <ehem+xen@xxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> ---
> v2: Also adjust ept_sync_domain_prepare(). Also convert the flush at the
>     out_of_memory label. Extend description to cover these.
> 
> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -1253,7 +1253,7 @@ static void ept_sync_domain_prepare(stru
>      {
>          if ( p2m_is_nestedp2m(p2m) )
>              ept = &p2m_get_hostp2m(d)->ept;
> -        else
> +        else if ( !p2m->defer_nested_flush )
>              p2m_flush_nestedp2m(d);
>      }
> 
> --- 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);
> @@ -1229,22 +1242,25 @@ 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);
> +    pod_unlock_and_flush(p2m);
> 
>      printk("%s: Dom%d out of PoD memory! (tot=%"PRIu32" ents=%ld
> dom%d)\n",
>             __func__, d->domain_id, domain_tot_pages(d),
>             p2m->pod.entry_count, current->domain->domain_id);
>      domain_crash(d);
>      return false;
> +
>  out_fail:
> -    pod_unlock(p2m);
> +    pod_unlock_and_flush(p2m);
>      return false;
> +
>  remap_and_retry:
>      BUG_ON(order != PAGE_ORDER_2M);
> -    pod_unlock(p2m);
> +    pod_unlock_and_flush(p2m);
> 
>      /*
>       * Remap this 2-meg region in singleton chunks. See the comment on the