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

[Fabiano Rosas <farosas@linux.ibm.com>]

Nicholas Piggin <npiggin@gmail.com> writes:

> 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.

Do you intend to make the KVM capability return < 0 in case AIL_3 is not
supported? AFAICS the unknown capability won't result in an ioctl error
as kvm_vm_ioctl_check_extension always returns >= 0.

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