Intel CAT (Cache Allocation Technology) and similar helps a bit by subdividing the cache between trust zones. So if you have two VMs rented by different tenants and you have set up CAT to isolate them then they shouldn't be able to interfere with each other. Of course CAT only covers the lowest layer of cache so there may be still be attacks on stuff in L1/L2.

Edit: Paper on this subject: http://palms.ee.princeton.edu/system/files/CATalyst_vfinal_c...