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

Claudio Imbrenda
2024-11-25T12:20:51Z

On Mon, 25 Nov 2024 12:50:39 +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.

wouldn't an atomic bit_op be better in that case?

>
> 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;
> 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];