Re: [PATCH v2 3/3] target/ppc/kvm: Use KVM_CAP_PPC_AIL_MODE_3 to determine cap-ail-mode-3 support

[David Gibson]

On Wed, Feb 16, 2022 at 04:39:03PM +1000, Nicholas Piggin wrote:
> 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>

LGTM, once the kernel side work is done.

> ---
> Since v1: 
> - Remove incorrect test for unsupported cap query. Add comment instead.
> 
>  linux-headers/linux/kvm.h |  1 +
>  target/ppc/kvm.c          | 18 ++++++++++++++++++
>  2 files changed, 19 insertions(+)
> 
> 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

You may be aware of this already, but once the cap is allocated on the
kernel side, you should do a full update of the kernel headers as a
separate patch.  A direct hack to the headers copy is fine for a
preliminary posting, though.

>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 1338c41f8f..7f71f78253 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;
> @@ -2567,6 +2569,17 @@ bool kvmppc_supports_ail_3(void)
>  {
>      PowerPCCPUClass *pcc = kvm_ppc_get_host_cpu_class();
>  
> +    if (cap_ail_mode_3) {
> +        return 1;
> +    }
> +
> +    /*
> +     * cap-ail-mode-3 is disabled, but it may be because the KVM host 
> pre-dates
> +     * the cap. Special-case the test because the performance cost for
> +     * disabling the feature unconditionally is prohibitive until updated
> +     * KVM is widely in use.
> +     */
> +
>      /*
>       * KVM PR only supports AIL-0
>       */
> @@ -2589,6 +2602,11 @@ bool kvmppc_supports_ail_3(void)
>          return 0;
>      }
>  
> +    /*
> +     * Beyond ISA v3.1 (POWER10), this could return 0, because all KVM
> +     * implementations for such hosts would support the cap.
> +     */
> +
>      return 1;
>  }
>  

-- 
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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