Re: [PATCH 2/5] ppc/spapr: Improve FWNMI machine check delivery corner case comments

[Greg Kurz <groug@kaod.org>]

On Tue, 17 Mar 2020 15:02:12 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> Some of the conditions are not as clearly documented as they could be.
> Also the non-FWNMI case does not need a large comment.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---

LGTM

Reviewed-by: Greg Kurz <groug@kaod.org>

>  hw/ppc/spapr_events.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c
> index 323fcef4aa..05337f0671 100644
> --- a/hw/ppc/spapr_events.c
> +++ b/hw/ppc/spapr_events.c
> @@ -834,17 +834,13 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>      Error *local_err = NULL;
>  
>      if (spapr->fwnmi_machine_check_addr == -1) {
> -        /*
> -         * This implies that we have hit a machine check either when the
> -         * guest has not registered FWNMI (i.e., "ibm,nmi-register" not
> -         * called) or between system reset and "ibm,nmi-register".
> -         * Fall back to the old machine check behavior in such cases.
> -         */
> +        /* Non-FWNMI case, deliver it like an architected CPU interrupt. */
>          cs->exception_index = POWERPC_EXCP_MCHECK;
>          ppc_cpu_do_interrupt(cs);
>          return;
>      }
>  
> +    /* Wait for FWNMI interlock. */
>      while (spapr->fwnmi_machine_check_interlock != -1) {
>          /*
>           * Check whether the same CPU got machine check error
> @@ -856,8 +852,13 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>              return;
>          }
>          qemu_cond_wait_iothread(&spapr->fwnmi_machine_check_interlock_cond);
> -        /* Meanwhile if the system is reset, then just return */
>          if (spapr->fwnmi_machine_check_addr == -1) {
> +            /*
> +             * If the machine was reset while waiting for the interlock,
> +             * abort the delivery. The machine check applies to a context
> +             * that no longer exists, so it wouldn't make sense to deliver
> +             * it now.
> +             */
>              return;
>          }
>      }
> @@ -868,7 +869,9 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>           * We don't want to abort so we let the migration to continue.
>           * In a rare case, the machine check handler will run on the target.
>           * Though this is not preferable, it is better than aborting
> -         * the migration or killing the VM.
> +         * the migration or killing the VM. It is okay to call
> +         * migrate_del_blocker on a blocker that was not added (which the
> +         * nmi-interlock handler would do when it's called after this).
>           */
>          warn_report("Received a fwnmi while migration was in progress");
>      }