linux-kernel - Re: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for HPT resizing

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Date:   Fri, 16 Dec 2016 14:15:30 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     David Gibson <david@...son.dropbear.id.au>, paulus@...ba.org
Cc:     michael@...erman.id.au, benh@...nel.crashing.org,
        sjitindarsingh@...il.com, lvivier@...hat.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for
 HPT resizing

On 15.12.2016 06:53, David Gibson wrote:
> This adds a new powerpc-specific KVM_CAP_SPAPR_RESIZE_HPT capability to
> advertise whether KVM is capable of handling the PAPR extensions for
> resizing the hashed page table during guest runtime.
> 
> At present, HPT resizing is possible with KVM PR without kernel
> modification, since the HPT is managed within qemu.  It's not possible yet
> with KVM HV, because the HPT is managed by KVM.  At present, qemu has to
> use other capabilities which (by accident) reveal whether PR or HV is in
> use to know if it can advertise HPT resizing capability to the guest.
> 
> To avoid ambiguity with existing kernels, the encoding is a bit odd.
>     0 means "unknown" since that's what previous kernels will return
>     1 means "HPT resize possible if available if and only if the HPT is allocated in
>       userspace, rather than in the kernel".  Userspace can check
>       KVM_CAP_PPC_ALLOC_HTAB to determine if that's the case.  In practice
>       this will give the same results as userspace's fallback check.
>     2 will mean "HPT resize available and implemented via ioctl()s
>       KVM_PPC_RESIZE_HPT_PREPARE and KVM_PPC_RESIZE_HPT_COMMIT"
> 
> For now we always return 1, but the intention is to return 2 once HPT
> resize is implemented for KVM HV.
> 
> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> ---
>  arch/powerpc/kvm/powerpc.c |  3 +++
>  include/uapi/linux/kvm.h   | 10 ++++++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index efd1183..bb23923 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -605,6 +605,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_SPAPR_MULTITCE:
>  		r = 1;
>  		break;
> +	case KVM_CAP_SPAPR_RESIZE_HPT:
> +		r = 1; /* resize allowed only if HPT is outside kernel */
> +		break;
>  #endif
>  	case KVM_CAP_PPC_HTM:
>  		r = cpu_has_feature(CPU_FTR_TM_COMP) &&
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index cac48ed..904afe0 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -685,6 +685,12 @@ struct kvm_ppc_smmu_info {
>  	struct kvm_ppc_one_seg_page_size sps[KVM_PPC_PAGE_SIZES_MAX_SZ];
>  };
>  
> +/* for KVM_PPC_RESIZE_HPT_{PREPARE,COMMIT} */
> +struct kvm_ppc_resize_hpt {
> +	__u64 flags;
> +	__u32 shift;
> +};

I think you should also add a final "__u32 pad" to that struct to make
sure that it is naturally aligned (like it is done with struct
kvm_coalesced_mmio_zone already for example).

 Thomas