linux-kernel - Re: [PATCH v2 3/3] KVM: s390: Increase size of union sca

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Message-ID: <2d3862ea-4112-4a03-9e4b-ac4e8e23a7f4@linux.ibm.com>
Date: Tue, 26 Nov 2024 13:09:56 +0100
From: Janosch Frank <frankja@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>
Cc: kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] KVM: s390: Increase size of union sca_utility to
 four bytes

On 11/26/24 11:25 AM, Heiko Carstens wrote:
[...]
>   union sca_utility {

Would you mind adding a comment?


""Utility is defined as 2 bytes but having it 4 bytes wide generates 
more efficient code. Since the following bytes are reserved this makes 
no functional difference.""

> -	__u16 val;
> +	__u32 val;
>   	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];