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 <c...@fr.ibm.com>
---
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
