[Qemu-ppc] [RFC PATCH 2/2] target-ppc: fix interrupt vectors address migration

[Cédric Le Goater]

commit 2360b6e84f78 ("target-ppc: force update of msr bits in
cpu_post_load") introduced a change to restore env->excp_prefix of a
guest which could have altered its MSR_EP. To do this, cpu_post_load()
invalidates msr and then calls ppc_store_msr() with the expected value
in argument.

The problem is that ppc_store_msr() relies on a 'valid' current msr
before changing its value. The MSR_HVB and MSR_TGPR bits are excluded
from the msr reset to keep the checks valid but the MSR_IR, MSR_DR,
MSR_EP bits which are also used through the msr_{ir,dr,ep} macros, are
reseted.

This is an issue for CPUs not using MSR_EP, on the spapr platform for
instance but all book3s are impacted. If excp_prefix is restored to
some value, it will be reseted by this call, causing an ISEG exception
on spapr guests.

This patch proposal is to test the msr_mask before actually testing
the MSR_EP bit and protect excp_prefix.

Signed-off-by: Cédric Le Goater <address@hidden>
---

 Should we just move the test in cpu_post_load() and not reset MSR_EP
 if it is not present in msr_mask ? like this is done for MSR_HVB and
 MSR_TGPR. I think this is making assumptions on what ppc_store_msr()
 is up to though.

 Maybe we could add a POWERPC_FLAGS_ for this purpose ? or test the
 excp_model ?
 
 There is room for improvement in ppc_store_msr(). It might need a new
 helper like ppc_restore_msr() ?

 Suggestions welcomed.
 
 target-ppc/helper_regs.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/target-ppc/helper_regs.h b/target-ppc/helper_regs.h
index 271fddf17f0a..2a72e000ed83 100644
--- a/target-ppc/helper_regs.h
+++ b/target-ppc/helper_regs.h
@@ -92,9 +92,11 @@ static inline int hreg_store_msr(CPUPPCState *env, 
target_ulong value,
         /* Swap temporary saved registers with GPRs */
         hreg_swap_gpr_tgpr(env);
     }
-    if (unlikely((value >> MSR_EP) & 1) != msr_ep) {
-        /* Change the exception prefix on PowerPC 601 */
-        env->excp_prefix = ((value >> MSR_EP) & 1) * 0xFFF00000;
+    if ((env->msr_mask >> MSR_EP) & 1) {
+        if (unlikely((value >> MSR_EP) & 1) != msr_ep) {
+            /* Change the exception prefix on PowerPC 601 */
+            env->excp_prefix = ((value >> MSR_EP) & 1) * 0xFFF00000;
+        }
     }
 #endif
     env->msr = value;
-- 
2.1.4