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

[Peter Maydell]

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 ?

thanks
-- PMM