target/ppc: Fix slbia TLB invalidation gap

slbia must invalidate TLBs even if it does not remove a valid SLB
entry, because slbmte can overwrite valid entries without removing
their TLBs.

As the architecture says, slbia invalidates all lookaside information,
not conditionally based on if it removed valid entries.

It does not seem possible for POWER8 or earlier Linux kernels to hit
this bug because it never changes its kernel SLB translations, and it
should always have valid entries if any accesses are made to userspace
regions. However other operating systems which may modify SLB entry 0
or do more fancy things with segments might be affected.

When POWER9 slbia support is added in the next patch, this becomes a
real problem because some new slbia variants don't invalidate all
non-zero entries.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20200318044135.851716-1-npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

diff --git a/target/ppc/mmu-hash64.c b/target/ppc/mmu-hash64.c
index 34f6009b1e5f70ba219004210a6a3700a3d66682..373d44de744cca0bd43eac114fc77e404d84f94e 100644 (file)
--- a/target/ppc/mmu-hash64.c
+++ b/target/ppc/mmu-hash64.c
@@ -100,20 +100,29 @@ void helper_slbia(CPUPPCState *env)
     PowerPCCPU *cpu = env_archcpu(env);
     int n;
 
+    /*
+     * slbia must always flush all TLB (which is equivalent to ERAT in ppc
+     * architecture). Matching on SLB_ESID_V is not good enough, because slbmte
+     * can overwrite a valid SLB without flushing its lookaside information.
+     *
+     * It would be possible to keep the TLB in synch with the SLB by flushing
+     * when a valid entry is overwritten by slbmte, and therefore slbia would
+     * not have to flush unless it evicts a valid SLB entry. However it is
+     * expected that slbmte is more common than slbia, and slbia is usually
+     * going to evict valid SLB entries, so that tradeoff is unlikely to be a
+     * good one.
+     */
+
     /* XXX: Warning: slbia never invalidates the first segment */
     for (n = 1; n < cpu->hash64_opts->slb_size; n++) {
         ppc_slb_t *slb = &env->slb[n];
 
         if (slb->esid & SLB_ESID_V) {
             slb->esid &= ~SLB_ESID_V;
-            /*
-             * XXX: given the fact that segment size is 256 MB or 1TB,
-             *      and we still don't have a tlb_flush_mask(env, n, mask)
-             *      in QEMU, we just invalidate all TLBs
-             */
-            env->tlb_need_flush |= TLB_NEED_LOCAL_FLUSH;
         }
     }
+
+    env->tlb_need_flush |= TLB_NEED_LOCAL_FLUSH;
 }
 
 static void __helper_slbie(CPUPPCState *env, target_ulong addr,