[POWERPC] spufs: invalidate SLB translation before adding a new entry

When we replace an SLB entry in the MFC after using up all the available
entries, there is a short window in which an incorrect entry is marked
as valid.

The problem is that the 'valid' bit is stored in the ESID, which is
always written after the VSID. Overwriting the VSID first will make the
original ESID entry point to the new VSID, which means that any
concurrent DMA accessing the old ESID ends up being redirected to the
new virtual address.  A few cycles later, we write the new ESID and
everything is fine again.

That race can be closed by writing a zero entry to the ESID first, which
makes sure that the VSID is not accessed until we write the new ESID.

Note that we don't actually need to invalidate the SLB entry using the
invalidation register, which would also flush any ERAT entries for that
segment, because the segment translation does not become invalid but is
only removed from the SLB cache.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

arch/powerpc/platforms/cell/spu_base.c

@@ -148,7 +148,11 @@ static inline void spu_load_slb(struct spu *spu, int slbe, struct spu_slb *slb)
 			__func__, slbe, slb->vsid, slb->esid);
 
 	out_be64(&priv2->slb_index_W, slbe);
+	/* set invalid before writing vsid */
+	out_be64(&priv2->slb_esid_RW, 0);
+	/* now it's safe to write the vsid */
 	out_be64(&priv2->slb_vsid_RW, slb->vsid);
+	/* setting the new esid makes the entry valid again */
 	out_be64(&priv2->slb_esid_RW, slb->esid);
 }