Re: [PATCH 1/5] ppc/spapr: KVM FWNMI should not be enabled until guest requests it

[Greg Kurz <groug@kaod.org>]

On Tue, 17 Mar 2020 15:02:11 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> The KVM FWNMI capability should be enabled with the "ibm,nmi-register"
> rtas call. Although MCEs from KVM will be delivered as architected
> interrupts to the guest before "ibm,nmi-register" is called, KVM has
> different behaviour depending on whether the guest has enabled FWNMI
> (it attempts to do more recovery on behalf of a non-FWNMI guest).
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  hw/ppc/spapr_caps.c  | 5 +++--
>  hw/ppc/spapr_rtas.c  | 7 +++++++
>  target/ppc/kvm.c     | 7 +++++++
>  target/ppc/kvm_ppc.h | 6 ++++++
>  4 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> index 679ae7959f..eb5521d0c2 100644
> --- a/hw/ppc/spapr_caps.c
> +++ b/hw/ppc/spapr_caps.c
> @@ -517,9 +517,10 @@ static void cap_fwnmi_apply(SpaprMachineState *spapr, uint8_t val,
>      }
>  
>      if (kvm_enabled()) {
> -        if (kvmppc_set_fwnmi() < 0) {
> +        if (!kvmppc_get_fwnmi()) {
>              error_setg(errp, "Firmware Assisted Non-Maskable Interrupts(FWNMI) "
> -                             "not supported by KVM");
> +                             "not supported by KVM, "
> +                             "try appending -machine cap-fwnmi=off");

It is usually preferred to keep error message strings on one
line for easier grepping. Also hints should be specified with
error_append_hint() because they are treated differently by
QMP (ie. not printed).

Something like:

        if (!kvmppc_get_fwnmi()) {
            error_setg(errp,
       "Firmware Assisted Non-Maskable Interrupts(FWNMI) not supported by KVM");
            error_append_hint(errp, "Try appending -machine cap-fwnmi=off\n");
        }

Note that the current error handling code has an issue that
prevents hints to be printed when errp == &error_fatal, which
is exactly what spapr_caps_apply() does. Since this affects
a lot of locations in the code base, there's an on-going
effort to fix that globally:

https://patchwork.ozlabs.org/project/qemu-devel/list/?series=163907

I don't know if this will make it for 5.0, but in any case I
think you should call error_append_hint() in this patch anyway
and the code will just work at some later point.

Rest looks good.

>          }
>      }
>  }
> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> index 9fb8c8632a..29abe66d01 100644
> --- a/hw/ppc/spapr_rtas.c
> +++ b/hw/ppc/spapr_rtas.c
> @@ -437,6 +437,13 @@ static void rtas_ibm_nmi_register(PowerPCCPU *cpu,
>          return;
>      }
>  
> +    if (kvm_enabled()) {
> +        if (kvmppc_set_fwnmi() < 0) {
> +            rtas_st(rets, 0, RTAS_OUT_NOT_SUPPORTED);
> +            return;
> +        }
> +    }
> +
>      spapr->fwnmi_system_reset_addr = sreset_addr;
>      spapr->fwnmi_machine_check_addr = mce_addr;
>  
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 597f72be1b..03d0667e8f 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -88,6 +88,7 @@ static int cap_ppc_safe_indirect_branch;
>  static int cap_ppc_count_cache_flush_assist;
>  static int cap_ppc_nested_kvm_hv;
>  static int cap_large_decr;
> +static int cap_fwnmi;
>  
>  static uint32_t debug_inst_opcode;
>  
> @@ -136,6 +137,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>      kvmppc_get_cpu_characteristics(s);
>      cap_ppc_nested_kvm_hv = kvm_vm_check_extension(s, KVM_CAP_PPC_NESTED_HV);
>      cap_large_decr = kvmppc_get_dec_bits();
> +    cap_fwnmi = kvm_vm_check_extension(s, KVM_CAP_PPC_FWNMI);
>      /*
>       * Note: setting it to false because there is not such capability
>       * in KVM at this moment.
> @@ -2064,6 +2066,11 @@ void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
>      }
>  }
>  
> +bool kvmppc_get_fwnmi(void)
> +{
> +    return cap_fwnmi;
> +}
> +
>  int kvmppc_set_fwnmi(void)
>  {
>      PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
> diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> index 332fa0aa1c..fcaf745516 100644
> --- a/target/ppc/kvm_ppc.h
> +++ b/target/ppc/kvm_ppc.h
> @@ -27,6 +27,7 @@ void kvmppc_enable_h_page_init(void);
>  void kvmppc_set_papr(PowerPCCPU *cpu);
>  int kvmppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr);
>  void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy);
> +bool kvmppc_get_fwnmi(void);
>  int kvmppc_set_fwnmi(void);
>  int kvmppc_smt_threads(void);
>  void kvmppc_error_append_smt_possible_hint(Error *const *errp);
> @@ -163,6 +164,11 @@ static inline void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
>  {
>  }
>  
> +static inline bool kvmppc_get_fwnmi(void)
> +{
> +    return false;
> +}
> +
>  static inline int kvmppc_set_fwnmi(void)
>  {
>      return -1;