Re: [PATCH 20/20] MIPS: KVM: Wire up MSA capability

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/03/2015 15:44, James Hogan wrote:
> Now that the code is in place for KVM to support MIPS SIMD Architecutre
> (MSA) in MIPS guests, wire up the new KVM_CAP_MIPS_MSA capability.
> 
> For backwards compatibility, the capability must be explicitly enabled
> in order to detect or make use of MSA from the guest.
> 
> The capability is not supported if the hardware supports MSA vector
> partitioning, since the extra support cannot be tested yet and it
> extends the state that the userland program would have to save.
> 
> Signed-off-by: James Hogan <james.hogan@imgtec.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Gleb Natapov <gleb@kernel.org>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: linux-mips@linux-mips.org
> Cc: kvm@vger.kernel.org
> Cc: linux-api@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Signed-off-by: James Hogan <james.hogan@imgtec.com>
> ---
>  Documentation/virtual/kvm/api.txt | 12 ++++++++++++
>  arch/mips/kvm/mips.c              | 24 ++++++++++++++++++++++++
>  include/uapi/linux/kvm.h          |  1 +
>  3 files changed, 37 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 47ddf0475211..97dd9ee69ca8 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -3224,6 +3224,18 @@ done the KVM_REG_MIPS_FPR_* and KVM_REG_MIPS_FCR_* registers can be accessed
>  Config5.FRE bits are accessible via the KVM API and also from the guest,
>  depending on them being supported by the FPU.
>  
> +6.10 KVM_CAP_MIPS_MSA
> +
> +Architectures: mips
> +Target: vcpu
> +Parameters: args[0] is reserved for future use (should be 0).
> +
> +This capability allows the use of the MIPS SIMD Architecture (MSA) by the guest.
> +It allows the Config3.MSAP bit to be set to enable the use of MSA by the guest.
> +Once this is done the KVM_REG_MIPS_VEC_* and KVM_REG_MIPS_MSA_* registers can be
> +accessed, and the Config5.MSAEn bit is accessible via the KVM API and also from
> +the guest.
> +
>  7. Capabilities that can be enabled on VMs
>  ------------------------------------------
>  
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index 9319c4360285..3b3530f493eb 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -880,6 +880,15 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
>  			return -EINVAL;
>  		vcpu->arch.fpu_enabled = true;
>  		break;
> +	case KVM_CAP_MIPS_MSA:
> +		/*
> +		 * MSA vector partitioning not supported,
> +		 * see kvm_vm_ioctl_check_extension().
> +		 */
> +		if (!cpu_has_msa || boot_cpu_data.msa_id & MSA_IR_WRPF)
> +			return -EINVAL;

Perhaps you can call kvm_vm_ioctl_check_extension directly, outside the
switch (it's okay if it's called for a capability other than FPU and MSA)?

Apart from this nit,

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

> +		vcpu->arch.msa_enabled = true;
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;
> @@ -1071,6 +1080,21 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_MIPS_FPU:
>  		r = !!cpu_has_fpu;
>  		break;
> +	case KVM_CAP_MIPS_MSA:
> +		/*
> +		 * We don't support MSA vector partitioning yet:
> +		 * 1) It would require explicit support which can't be tested
> +		 *    yet due to lack of support in current hardware.
> +		 * 2) It extends the state that would need to be saved/restored
> +		 *    by e.g. QEMU for migration.
> +		 *
> +		 * When vector partitioning hardware becomes available, support
> +		 * could be added by requiring a flag when enabling
> +		 * KVM_CAP_MIPS_MSA capability to indicate that userland knows
> +		 * to save/restore the appropriate extra state.
> +		 */
> +		r = cpu_has_msa && !(boot_cpu_data.msa_id & MSA_IR_WRPF);
> +		break;
>  	default:
>  		r = 0;
>  		break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 98f6e5c653ff..5f859888e3ad 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -761,6 +761,7 @@ struct kvm_ppc_smmu_info {
>  #define KVM_CAP_CHECK_EXTENSION_VM 105
>  #define KVM_CAP_S390_USER_SIGP 106
>  #define KVM_CAP_MIPS_FPU 107
> +#define KVM_CAP_MIPS_MSA 108
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
>