Re: [PATCH v2 3/3] KVM: s390: Increase size of union sca_utility to four bytes

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 26 Nov 2024 11:25:15 +0100
Heiko Carstens <hca@linux.ibm.com> wrote:

> kvm_s390_update_topology_change_report() modifies a single bit within
> sca_utility using cmpxchg(). Given that the size of the sca_utility union
> is two bytes this generates very inefficient code. Change the size to four
> bytes, so better code can be generated.
> 
> Even though the size of sca_utility doesn't reflect architecture anymore
> this seems to be the easiest and most pragmatic approach to avoid
> inefficient code.
> 
> Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/include/asm/kvm_host.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 1cd8eaebd3c0..1cb1de232b9e 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -95,10 +95,10 @@ union ipte_control {
>  };
>  
>  union sca_utility {
> -	__u16 val;
> +	__u32 val;

I know I said the patch was fine but I realised now that I would like a
short comment here explaining that 32 bits allows for more efficient
code

you can add it when picking, no need to send a v3

>  	struct {
> -		__u16 mtcr : 1;
> -		__u16 reserved : 15;
> +		__u32 mtcr : 1;
> +		__u32	   : 31;
>  	};
>  };
>  
> @@ -107,7 +107,7 @@ struct bsca_block {
>  	__u64	reserved[5];
>  	__u64	mcn;
>  	union sca_utility utility;
> -	__u8	reserved2[6];
> +	__u8	reserved2[4];
>  	struct bsca_entry cpu[KVM_S390_BSCA_CPU_SLOTS];
>  };
>  
> @@ -115,7 +115,7 @@ struct esca_block {
>  	union ipte_control ipte_control;
>  	__u64   reserved1[6];
>  	union sca_utility utility;
> -	__u8	reserved2[6];
> +	__u8	reserved2[4];
>  	__u64   mcn[4];
>  	__u64   reserved3[20];
>  	struct esca_entry cpu[KVM_S390_ESCA_CPU_SLOTS];