On Mon, Mar 21, 2016 at 03:02:08PM +0100, Cédric Le Goater wrote:
> 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.
So, IIUC, in spapr MSR[EP] can't be set with mtmsr or rfid, but can be
set with H_SET_MODE?
I think what we need to do is to make sure the full MSR value is
migrated then, even if EP is not in the msr_mask. Once that's done,
we should be able to correctly calculate excp_prefix from the MSR
value after migration.
Or am I missing something?
> 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;
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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