linux-kernel - Re: [PATCH 1/7] powerpc/watchpoint/kvm: Rename current DAWR macros and variables

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Date:   Wed, 2 Sep 2020 11:49:08 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     mpe@...erman.id.au, mikey@...ling.org, npiggin@...il.com,
        pbonzini@...hat.com, christophe.leroy@....fr, jniethe5@...il.com,
        pedromfc@...ibm.com, rogealve@...ibm.com, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/7] powerpc/watchpoint/kvm: Rename current DAWR macros
 and variables

On Thu, Jul 23, 2020 at 03:50:52PM +0530, Ravi Bangoria wrote:
> Power10 is introducing second DAWR. Use real register names (with
> suffix 0) from ISA for current macros and variables used by kvm.

Most of this looks fine, but I think we should not change the existing
names in arch/powerpc/include/uapi/asm/kvm.h (and therefore also
Documentation/virt/kvm/api.rst).

> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 426f94582b7a..4dc18fe6a2bf 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -2219,8 +2219,8 @@ registers, find a list below:
>    PPC     KVM_REG_PPC_BESCR               64
>    PPC     KVM_REG_PPC_TAR                 64
>    PPC     KVM_REG_PPC_DPDES               64
> -  PPC     KVM_REG_PPC_DAWR                64
> -  PPC     KVM_REG_PPC_DAWRX               64
> +  PPC     KVM_REG_PPC_DAWR0               64
> +  PPC     KVM_REG_PPC_DAWRX0              64
>    PPC     KVM_REG_PPC_CIABR               64
>    PPC     KVM_REG_PPC_IC                  64
>    PPC     KVM_REG_PPC_VTB                 64
...
> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
> index 264e266a85bf..38d61b73f5ed 100644
> --- a/arch/powerpc/include/uapi/asm/kvm.h
> +++ b/arch/powerpc/include/uapi/asm/kvm.h
> @@ -608,8 +608,8 @@ struct kvm_ppc_cpu_char {
>  #define KVM_REG_PPC_BESCR	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xa7)
>  #define KVM_REG_PPC_TAR		(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xa8)
>  #define KVM_REG_PPC_DPDES	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xa9)
> -#define KVM_REG_PPC_DAWR	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xaa)
> -#define KVM_REG_PPC_DAWRX	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xab)
> +#define KVM_REG_PPC_DAWR0	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xaa)
> +#define KVM_REG_PPC_DAWRX0	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xab)
>  #define KVM_REG_PPC_CIABR	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xac)
>  #define KVM_REG_PPC_IC		(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xad)
>  #define KVM_REG_PPC_VTB		(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xae)

The existing names are an API, and if you change them you will break
compilation of existing userspace programs.  I don't see that adding
the '0' on the end is so important that we need to break userspace.

Paul.