| Message ID | 20241126102515.3178914-4-hca@linux.ibm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: s390: Couple of small cmpxchg() optimizations | expand |
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];
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];
On Tue, 26 Nov 2024 13:09:56 +0100 Janosch Frank <frankja@linux.ibm.com> wrote: > 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."" looks good, thanks! > > > - __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 Tue, Nov 26, 2024 at 01:21:52PM +0100, Claudio Imbrenda wrote: > On Tue, 26 Nov 2024 13:09:56 +0100 > Janosch Frank <frankja@linux.ibm.com> wrote: > > > 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."" > > looks good, thanks! Thanks a lot! I added the comment and applied the series.
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];