Re: [PATCH v2] KVM: PPC: Enable CAP_SPAPR_TCE_VFIO on pSeries KVM guests

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> Currently on book3s-hv, the capability KVM_CAP_SPAPR_TCE_VFIO is only
> available for KVM Guests running on PowerNV and not for the KVM guests
> running on pSeries hypervisors. This prevents a pSeries L2 guest from
> leveraging the in-kernel acceleration for H_PUT_TCE_INDIRECT and
> H_STUFF_TCE hcalls that results in slow startup times for large memory
> guests.
>
> Fix this by enabling the CAP_SPAPR_TCE_VFIO on the pSeries hosts as well
> for the nested PAPR guests. With the patch, booting an L2 guest with
> 128G memory results in an average improvement of 11% in the startup
> times.
>
> Fixes: f431a8cde7f1 ("powerpc/iommu: Reimplement the iommu_table_group_ops for pSeries")
> Cc: stable@vger.kernel.org
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> Changes since v1:
>     * Addressed review comments from Ritesh
>     * v1: https://lore.kernel.org/all/20250109132053.158436-1-amachhiw@linux.ibm.com/

Thanks Amit for v2. However we still didn't answer one important
question regarding the context / background of this patch asked here [2]

[2]: https://lore.kernel.org/linuxppc-dev/87r059vpmi.fsf@gmail.com/

<copy paste from v1>
    IIUC it was said here [1] that this capability is not available on
    pSeries, hence it got removed. Could you please give a background on
    why this can be enabled now for pSeries? Was there any additional
    support added for this? 
    [1]:
    https://lore.kernel.org/linuxppc-dev/20181214052910.23639-2-sjitindarsingh@gmail.com/

    ... Ohh thinking back a little, are you saying that after the patch...
    f431a8cde7f1 ("powerpc/iommu: Reimplement the iommu_table_group_ops for pSeries")
    ...we can bring back this capability for kvm guest running on pseries
    as well. Because all underlying issues in using VFIO on pseries were
    fixed. Is this understanding correct? 


Please also update the commit message with the required context of why we can
enable this capability now while it was explicitely marked as disabled
earlier in [1].

But looks good otherwise. With that addressed in the commit message,
please feel free to add - 

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

-ritesh

>
>  arch/powerpc/kvm/powerpc.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index ce1d91eed231..a7138eb18d59 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -543,26 +543,23 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		r = !hv_enabled;
>  		break;
>  #ifdef CONFIG_KVM_MPIC
>  	case KVM_CAP_IRQ_MPIC:
>  		r = 1;
>  		break;
>  #endif
>
>  #ifdef CONFIG_PPC_BOOK3S_64
>  	case KVM_CAP_SPAPR_TCE:
> +		fallthrough;
>  	case KVM_CAP_SPAPR_TCE_64:
> -		r = 1;
> -		break;
>  	case KVM_CAP_SPAPR_TCE_VFIO:
> -		r = !!cpu_has_feature(CPU_FTR_HVMODE);
> -		break;
>  	case KVM_CAP_PPC_RTAS:
>  	case KVM_CAP_PPC_FIXUP_HCALL:
>  	case KVM_CAP_PPC_ENABLE_HCALL:
>  #ifdef CONFIG_KVM_XICS
>  	case KVM_CAP_IRQ_XICS:
>  #endif
>  	case KVM_CAP_PPC_GET_CPU_CHAR:
>  		r = 1;
>  		break;
>  #ifdef CONFIG_KVM_XIVE
>
> base-commit: 6d61a53dd6f55405ebcaea6ee38d1ab5a8856c2c
> -- 
> 2.48.1