Re: [PATCH 2/2] target/ppc/kvm: Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Fabiano Rosas's message of February 14, 2022 11:13 pm:
> Nicholas Piggin <npiggin@gmail.com> writes:
> 
>> Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support for KVM
>> guests. Keep the fallback heuristic for KVM hosts that pre-date this
>> CAP.
>>
>> This is only proposed the KVM CAP has not yet been allocated. I will
>> ask to merge the new KVM cap when there are no objections on the QEMU
>> side.
>>
>> not-yet-Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>>  hw/ppc/spapr_caps.c       |  2 +-
>>  linux-headers/linux/kvm.h |  1 +
>>  target/ppc/kvm.c          | 18 +++++++++++++++++-
>>  target/ppc/kvm_ppc.h      |  4 ++--
>>  4 files changed, 21 insertions(+), 4 deletions(-)
>>
>> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
>> index 5fd4a53c33..5cc80776d0 100644
>> --- a/hw/ppc/spapr_caps.c
>> +++ b/hw/ppc/spapr_caps.c
>> @@ -619,7 +619,7 @@ static void cap_ail_mode_3_apply(SpaprMachineState *spapr,
>>      ERRP_GUARD();
>>  
>>      if (kvm_enabled()) {
>> -        if (!kvmppc_supports_ail_3()) {
>> +        if (!kvmppc_has_cap_ail_3()) {
>>              error_setg(errp, "KVM implementation does not support cap-ail-mode-3");
>>              error_append_hint(errp, "Try appending -machine cap-ail-mode-3=off\n");
>>              return;
>> diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
>> index 02c5e7b7bb..d91f578200 100644
>> --- a/linux-headers/linux/kvm.h
>> +++ b/linux-headers/linux/kvm.h
>> @@ -1130,6 +1130,7 @@ struct kvm_ppc_resize_hpt {
>>  #define KVM_CAP_BINARY_STATS_FD 203
>>  #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
>>  #define KVM_CAP_ARM_MTE 205
>> +#define KVM_CAP_PPC_AIL_MODE_3 210
>>  
>>  #ifdef KVM_CAP_IRQ_ROUTING
>>  
>> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
>> index 128bc530d4..d0d0bdaac4 100644
>> --- a/target/ppc/kvm.c
>> +++ b/target/ppc/kvm.c
>> @@ -90,6 +90,7 @@ static int cap_ppc_nested_kvm_hv;
>>  static int cap_large_decr;
>>  static int cap_fwnmi;
>>  static int cap_rpt_invalidate;
>> +static int cap_ail_mode_3;
>>  
>>  static uint32_t debug_inst_opcode;
>>  
>> @@ -154,6 +155,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>>      }
>>  
>>      cap_rpt_invalidate = kvm_vm_check_extension(s, KVM_CAP_PPC_RPT_INVALIDATE);
>> +    cap_ail_mode_3 = kvm_vm_check_extension(s, KVM_CAP_PPC_AIL_MODE_3);
>>      kvm_ppc_register_host_cpu_type();
>>  
>>      return 0;
>> @@ -2563,10 +2565,24 @@ int kvmppc_has_cap_rpt_invalidate(void)
>>      return cap_rpt_invalidate;
>>  }
>>  
>> -int kvmppc_supports_ail_3(void)
>> +int kvmppc_has_cap_ail_3(void)
>>  {
>>      PowerPCCPUClass *pcc = kvm_ppc_get_host_cpu_class();
>>  
>> +    if (cap_ail_mode_3) {
>> +        return 1;
>> +    }
>> +
>> +    if (kvm_ioctl(kvm_state, KVM_CHECK_EXTENSION, KVM_CAP_PPC_AIL_MODE_3) == 0) {
>> +        return 0;
>> +    }
> 
> This is not needed here it seems.

This is to test whether the capability is recognised by the HV. 
kvm_vm_check_extension() treats ioctl error as 0 capability but we want 
to do this extra heuristic.

I'm not sure if there's a better standard way to do this.

Thanks,
Nick