Re: [PATCH for 8.1] intel_iommu: refine iotlb hash calculation

[Peter Xu <peterx@redhat.com>]

On Tue, Apr 11, 2023 at 03:30:08PM +0100, Peter Maydell wrote:
> On Tue, 11 Apr 2023 at 15:14, Peter Xu <peterx@redhat.com> wrote:
> >
> > On Mon, Apr 10, 2023 at 11:32:08AM +0800, Jason Wang wrote:
> > > @@ -222,9 +222,9 @@ static guint vtd_iotlb_hash(gconstpointer v)
> > >  {
> > >      const struct vtd_iotlb_key *key = v;
> > >
> > > -    return key->gfn | ((key->sid) << VTD_IOTLB_SID_SHIFT) |
> > > -           (key->level) << VTD_IOTLB_LVL_SHIFT |
> > > -           (key->pasid) << VTD_IOTLB_PASID_SHIFT;
> > > +    return key->gfn | ((uint64_t)(key->sid) << VTD_IOTLB_SID_SHIFT) |
> > > +        (uint64_t)(key->level - 1) << VTD_IOTLB_LVL_SHIFT |
> > > +        (uint64_t)(key->pasid) << VTD_IOTLB_PASID_SHIFT;
> > >  }
> 
> > >  /* The shift of source_id in the key of IOTLB hash table */
> > > -#define VTD_IOTLB_SID_SHIFT         20
> > > -#define VTD_IOTLB_LVL_SHIFT         28
> > > -#define VTD_IOTLB_PASID_SHIFT       30
> > > +#define VTD_IOTLB_SID_SHIFT         26
> > > +#define VTD_IOTLB_LVL_SHIFT         42
> > > +#define VTD_IOTLB_PASID_SHIFT       44
> >
> > This is for the hash function only, IIUC it means anything over
> > sizeof(guint) will be ignored and not contributing anything to the hash
> > value being generated due to the uint64->guint conversion.
> >
> > IOW, I think "level" and "pasid" will just be ignored.
> 
> Whoops, hadn't noticed that guint type... (glib's
> g_int64_hash()'s approach to this is to XOR the top
> 32 bits with the bottom 32 bits to produce the 32-bit
> hash value.)
> 
> Also, does anybody know what the requirements are on
> consistency between the hash_func and the key_equal_func
> for a GHashTable ? Is the hash_func supposed to return the
> same hash for every key that compares equal under key_equal_func ?

I quickly checked up a local (but old) glib code (v2.71.0), and it seems
this is the major place where key_equal_func() is used (also see the
comment above the comparison):

g_hash_table_lookup_node()
{
...
      /* We first check if our full hash values
       * are equal so we can avoid calling the full-blown
       * key equality function in most cases.
       */
      if (node_hash == hash_value)
        {
          gpointer node_key = g_hash_table_fetch_key_or_value (hash_table->keys, node_index, hash_table->have_big_keys);

          if (hash_table->key_equal_func)
            {
              if (hash_table->key_equal_func (node_key, key))
                return node_index;
            }
          else if (node_key == key)
            {
              return node_index;
            }
        }
...
}

I would guess hash_func() is only the fast version but if key_equal_func()
is provided it'll be the final / most accurate way to tell whether two
nodes are the same.

I assume from that POV the hash function should return the same value if
key_equal_func() tells they're the same node.

Thanks,

-- 
Peter Xu