| Message ID | 20241125115039.1809353-4-hca@linux.ibm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: s390: Couple of small cmpxchg() optimizations | expand |
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];
On Mon, Nov 25, 2024 at 01:20:42PM +0100, Claudio Imbrenda wrote: > 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? I had that, but decided against it, since the generated code isn't shorter. And it would require and unsigned long type within the union, or a cast, which I also both disliked.
On Mon, 25 Nov 2024 14:40:22 +0100 Heiko Carstens <hca@linux.ibm.com> wrote: > On Mon, Nov 25, 2024 at 01:20:42PM +0100, Claudio Imbrenda wrote: > > 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? > > I had that, but decided against it, since the generated code isn't shorter. > And it would require and unsigned long type within the union, or a cast, > which I also both disliked. fair enough Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
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];
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. Signed-off-by: Heiko Carstens <hca@linux.ibm.com> --- arch/s390/include/asm/kvm_host.h | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)