[v3 02/15] KVM: PPC: Book3S HV: Cleanup updates for kvm vcpu MMCR

[Paul Mackerras]

On Fri, Jul 17, 2020 at 10:38:14AM -0400, Athira Rajeev wrote:
> Currently `kvm_vcpu_arch` stores all Monitor Mode Control registers
> in a flat array in order: mmcr0, mmcr1, mmcra, mmcr2, mmcrs
> Split this to give mmcra and mmcrs its own entries in vcpu and
> use a flat array for mmcr0 to mmcr2. This patch implements this
> cleanup to make code easier to read.

Changing the way KVM stores these values internally is fine, but
changing the user ABI is not.  This part:

> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
> index 264e266..e55d847 100644
> --- a/arch/powerpc/include/uapi/asm/kvm.h
> +++ b/arch/powerpc/include/uapi/asm/kvm.h
> @@ -510,8 +510,8 @@ struct kvm_ppc_cpu_char {
>  
>  #define KVM_REG_PPC_MMCR0	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x10)
>  #define KVM_REG_PPC_MMCR1	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x11)
> -#define KVM_REG_PPC_MMCRA	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x12)
> -#define KVM_REG_PPC_MMCR2	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x13)
> +#define KVM_REG_PPC_MMCR2	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x12)
> +#define KVM_REG_PPC_MMCRA	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x13)

means that existing userspace programs that used to work would now be
broken.  That is not acceptable (breaking the user ABI is only ever
acceptable with a very compelling reason).  So NAK to this part of the
patch.

Regards,
Paul.