Re: [Qemu-devel] [RFC v3 3/3] ARM: KVM: Check KVM_CAP_ARM_IRQ_LINE_LAYOUT_2 for smp_cpus > 256

[Andrew Jones]

On Fri, Sep 13, 2019 at 11:56:39AM +0200, Eric Auger wrote:
> Host kernel within [4.18, 5.3] report an erroneous KVM_MAX_VCPUS=512
> for ARM. The actual capability to instantiate more than 256 vcpus
> was fixed in 5.4 with the upgrade of the KVM_IRQ_LINE ABI to support
> vcpu id encoded on 12 bits instead of 8 and a redistributor consuming
> a single KVM IO device instead of 2.
> 
> So let's check this capability when attempting to use more than 256
> vcpus within any ARM kvm accelerated machine.
> 
> Signed-off-by: Eric Auger <address@hidden>
> 
> ---
> 
> v2 -> v3:
> - Implement the check in kvm_arch_init as suggested by Peter
> ---
>  target/arm/kvm.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index b10581fa06..b473c63edb 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -182,6 +182,7 @@ int kvm_arm_get_max_vm_ipa_size(MachineState *ms)
>  
>  int kvm_arch_init(MachineState *ms, KVMState *s)
>  {
> +    int ret = 0;
>      /* For ARM interrupt delivery is always asynchronous,
>       * whether we are using an in-kernel VGIC or not.
>       */
> @@ -195,7 +196,14 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>  
>      cap_has_mp_state = kvm_check_extension(s, KVM_CAP_MP_STATE);
>  
> -    return 0;
> +    if (ms->smp.cpus > 256 &&
> +        !kvm_check_extension(s, KVM_CAP_ARM_IRQ_LINE_LAYOUT_2)) {
> +        error_report("Using more than 256 vcpus requires a host kernel "
> +                     "with KVM_CAP_ARM_IRQ_LINE_LAYOUT_2");
> +        ret = -EINVAL;
> +    }
> +
> +    return ret;
>  }
>  
>  unsigned long kvm_arch_vcpu_id(CPUState *cpu)
> -- 
> 2.20.1
> 
>

Reviewed-by: Andrew Jones <address@hidden>