Re: [PATCH] arm64: KVM: Add XXX UAPI notes for swapped registers

[Marc Zyngier <maz@kernel.org>]

Hi Andrew,

Many thanks for this. Comments below.

On 2020-01-20 13:08, Andrew Jones wrote:
> Two UAPI system register IDs do not derive their values from the
> ARM system register encodings. This is because their values were
> accidentally swapped. As the IDs are API, they cannot be changed.
> Add XXX notes to point them out.
> 
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  Documentation/virt/kvm/api.txt    |  8 ++++++++
>  arch/arm64/include/uapi/asm/kvm.h | 11 +++++++++--
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.txt 
> b/Documentation/virt/kvm/api.txt
> index ebb37b34dcfc..11556fc457c3 100644
> --- a/Documentation/virt/kvm/api.txt
> +++ b/Documentation/virt/kvm/api.txt
> @@ -2196,6 +2196,14 @@ arm64 CCSIDR registers are demultiplexed by 
> CSSELR value:
>  arm64 system registers have the following id bit patterns:
>    0x6030 0000 0013 <op0:2> <op1:3> <crn:4> <crm:4> <op2:3>
> 
> +XXX: Two system register IDs do not follow the specified pattern.  
> These
> +     are KVM_REG_ARM_TIMER_CVAL and KVM_REG_ARM_TIMER_CNT, which map 
> to
> +     system registers CNTV_CVAL_EL0 and CNTVCT_EL0 respectively.  
> These
> +     two had their values accidentally swapped, which means TIMER_CVAL 
> is
> +     derived from the register encoding for CNTVCT_EL0 and TIMER_CNT 
> is
> +     derived from the register encoding for CNTV_CVAL_EL0.  As this is
> +     API, it must remain this way.

Is 'XXX' an establiched way of documenting this kind of misfeature?
I couldn't find any other occurrence in Documentation, but I haven't
searched very hard.

If nobody has a better idea, I'll queue it as is.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...