Ping: [PATCH v4] x86/altp2m: p2m_altp2m_propagate_change() should honor present page order

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.02.2022 14:57, Jan Beulich wrote:
> For higher order mappings the comparison against p2m->min_remapped_gfn
> needs to take the upper bound of the covered GFN range into account, not
> just the base GFN. Otherwise, i.e. when dropping a mapping overlapping
> the remapped range but the base GFN outside of that range, an altp2m may
> wrongly not get reset.
> 
> Note that there's no need to call get_gfn_type_access() ahead of the
> check against the remapped range boundaries: None of its outputs are
> needed earlier, and p2m_reset_altp2m() doesn't require the lock to be
> held. In fact this avoids a latent lock order violation: With per-GFN
> locking p2m_reset_altp2m() not only doesn't require the GFN lock to be
> held, but holding such a lock would actually not be allowed, as the
> function acquires a P2M lock.
> 
> Local variables are moved into the more narrow scope (one is deleted
> altogether) to help see their actual life ranges.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Note that this addresses only half of the problem: get_gfn_type_access()
> would also need invoking for all of the involved GFNs, not just the 1st
> one.
> ---
> v4: Restore mistakenly dropped mfn_eq(mfn, INVALID_MFN).

I think this was the only open item I needed to deal with. Any chance
I could get an ack or R-b here, please?

Thanks, Jan

> v3: Don't pass the minimum of both orders to p2m_set_entry() (as was the
>     case in the original code). Restore get_gfn_type_access() return
>     value checking.
> 
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -2534,9 +2534,6 @@ int p2m_altp2m_propagate_change(struct d
>                                  p2m_type_t p2mt, p2m_access_t p2ma)
>  {
>      struct p2m_domain *p2m;
> -    p2m_access_t a;
> -    p2m_type_t t;
> -    mfn_t m;
>      unsigned int i;
>      unsigned int reset_count = 0;
>      unsigned int last_reset_idx = ~0;
> @@ -2549,15 +2546,17 @@ int p2m_altp2m_propagate_change(struct d
>  
>      for ( i = 0; i < MAX_ALTP2M; i++ )
>      {
> +        p2m_type_t t;
> +        p2m_access_t a;
> +
>          if ( d->arch.altp2m_eptp[i] == mfn_x(INVALID_MFN) )
>              continue;
>  
>          p2m = d->arch.altp2m_p2m[i];
> -        m = get_gfn_type_access(p2m, gfn_x(gfn), &t, &a, 0, NULL);
>  
>          /* Check for a dropped page that may impact this altp2m */
>          if ( mfn_eq(mfn, INVALID_MFN) &&
> -             gfn_x(gfn) >= p2m->min_remapped_gfn &&
> +             gfn_x(gfn) + (1UL << page_order) > p2m->min_remapped_gfn &&
>               gfn_x(gfn) <= p2m->max_remapped_gfn )
>          {
>              if ( !reset_count++ )
> @@ -2568,8 +2566,6 @@ int p2m_altp2m_propagate_change(struct d
>              else
>              {
>                  /* At least 2 altp2m's impacted, so reset everything */
> -                __put_gfn(p2m, gfn_x(gfn));
> -
>                  for ( i = 0; i < MAX_ALTP2M; i++ )
>                  {
>                      if ( i == last_reset_idx ||
> @@ -2583,16 +2579,19 @@ int p2m_altp2m_propagate_change(struct d
>                  break;
>              }
>          }
> -        else if ( !mfn_eq(m, INVALID_MFN) )
> +        else if ( !mfn_eq(get_gfn_type_access(p2m, gfn_x(gfn), &t, &a, 0,
> +                                              NULL), INVALID_MFN) )
>          {
>              int rc = p2m_set_entry(p2m, gfn, mfn, page_order, p2mt, p2ma);
>  
>              /* Best effort: Don't bail on error. */
>              if ( !ret )
>                  ret = rc;
> -        }
>  
> -        __put_gfn(p2m, gfn_x(gfn));
> +            __put_gfn(p2m, gfn_x(gfn));
> +        }
> +        else
> +            __put_gfn(p2m, gfn_x(gfn));
>      }
>  
>      altp2m_list_unlock(d);
> 
>