| Message ID | 20220223011331.12254-1-vincent.chen@sifive.com |
|---|---|
| State | Accepted |
| Headers | show |
| Series | [v2] RISC-V: KVM: Refine __kvm_riscv_switch_to() implementation | expand |
On Wed, Feb 23, 2022 at 6:43 AM Vincent Chen <vincent.chen@sifive.com> wrote: > > Kernel uses __kvm_riscv_switch_to() and __kvm_switch_return() to switch > the context of host kernel and guest kernel. Several CSRs belonging to the > context will be read and written during the context switch. To ensure > atomic read-modify-write control of CSR and ordering of CSR accesses, some > hardware blocks flush the pipeline when writing a CSR. In this > circumstance, grouping CSR executions together as much as possible can > reduce the performance impact of the pipeline. Therefore, this commit > reorders the CSR instructions to enhance the context switch performance.. > > Signed-off-by: Vincent Chen <vincent.chen@sifive.com> > Suggested-by: Hsinyi Lee <hsinyi.lee@sifive.com> > Suggested-by: Fu-Ching Yang <fu-ching.yang@sifive.com> I have queued this patch for 5.18 Thanks, Anup > --- > arch/riscv/kvm/vcpu_switch.S | 60 ++++++++++++++++++++---------------- > 1 file changed, 34 insertions(+), 26 deletions(-) > > diff --git a/arch/riscv/kvm/vcpu_switch.S b/arch/riscv/kvm/vcpu_switch.S > index 029a28a195c6..d74df8eb4d71 100644 > --- a/arch/riscv/kvm/vcpu_switch.S > +++ b/arch/riscv/kvm/vcpu_switch.S > @@ -41,33 +41,37 @@ ENTRY(__kvm_riscv_switch_to) > REG_S s10, (KVM_ARCH_HOST_S10)(a0) > REG_S s11, (KVM_ARCH_HOST_S11)(a0) > > - /* Save Host and Restore Guest SSTATUS */ > + /* Load Guest CSR values */ > REG_L t0, (KVM_ARCH_GUEST_SSTATUS)(a0) > + REG_L t1, (KVM_ARCH_GUEST_HSTATUS)(a0) > + REG_L t2, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) > + la t4, __kvm_switch_return > + REG_L t5, (KVM_ARCH_GUEST_SEPC)(a0) > + > + /* Save Host and Restore Guest SSTATUS */ > csrrw t0, CSR_SSTATUS, t0 > - REG_S t0, (KVM_ARCH_HOST_SSTATUS)(a0) > > /* Save Host and Restore Guest HSTATUS */ > - REG_L t1, (KVM_ARCH_GUEST_HSTATUS)(a0) > csrrw t1, CSR_HSTATUS, t1 > - REG_S t1, (KVM_ARCH_HOST_HSTATUS)(a0) > > /* Save Host and Restore Guest SCOUNTEREN */ > - REG_L t2, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) > csrrw t2, CSR_SCOUNTEREN, t2 > - REG_S t2, (KVM_ARCH_HOST_SCOUNTEREN)(a0) > - > - /* Save Host SSCRATCH and change it to struct kvm_vcpu_arch pointer */ > - csrrw t3, CSR_SSCRATCH, a0 > - REG_S t3, (KVM_ARCH_HOST_SSCRATCH)(a0) > > /* Save Host STVEC and change it to return path */ > - la t4, __kvm_switch_return > csrrw t4, CSR_STVEC, t4 > - REG_S t4, (KVM_ARCH_HOST_STVEC)(a0) > + > + /* Save Host SSCRATCH and change it to struct kvm_vcpu_arch pointer */ > + csrrw t3, CSR_SSCRATCH, a0 > > /* Restore Guest SEPC */ > - REG_L t0, (KVM_ARCH_GUEST_SEPC)(a0) > - csrw CSR_SEPC, t0 > + csrw CSR_SEPC, t5 > + > + /* Store Host CSR values */ > + REG_S t0, (KVM_ARCH_HOST_SSTATUS)(a0) > + REG_S t1, (KVM_ARCH_HOST_HSTATUS)(a0) > + REG_S t2, (KVM_ARCH_HOST_SCOUNTEREN)(a0) > + REG_S t3, (KVM_ARCH_HOST_SSCRATCH)(a0) > + REG_S t4, (KVM_ARCH_HOST_STVEC)(a0) > > /* Restore Guest GPRs (except A0) */ > REG_L ra, (KVM_ARCH_GUEST_RA)(a0) > @@ -145,32 +149,36 @@ __kvm_switch_return: > REG_S t5, (KVM_ARCH_GUEST_T5)(a0) > REG_S t6, (KVM_ARCH_GUEST_T6)(a0) > > + /* Load Host CSR values */ > + REG_L t1, (KVM_ARCH_HOST_STVEC)(a0) > + REG_L t2, (KVM_ARCH_HOST_SSCRATCH)(a0) > + REG_L t3, (KVM_ARCH_HOST_SCOUNTEREN)(a0) > + REG_L t4, (KVM_ARCH_HOST_HSTATUS)(a0) > + REG_L t5, (KVM_ARCH_HOST_SSTATUS)(a0) > + > /* Save Guest SEPC */ > csrr t0, CSR_SEPC > - REG_S t0, (KVM_ARCH_GUEST_SEPC)(a0) > - > - /* Restore Host STVEC */ > - REG_L t1, (KVM_ARCH_HOST_STVEC)(a0) > - csrw CSR_STVEC, t1 > > /* Save Guest A0 and Restore Host SSCRATCH */ > - REG_L t2, (KVM_ARCH_HOST_SSCRATCH)(a0) > csrrw t2, CSR_SSCRATCH, t2 > - REG_S t2, (KVM_ARCH_GUEST_A0)(a0) > + > + /* Restore Host STVEC */ > + csrw CSR_STVEC, t1 > > /* Save Guest and Restore Host SCOUNTEREN */ > - REG_L t3, (KVM_ARCH_HOST_SCOUNTEREN)(a0) > csrrw t3, CSR_SCOUNTEREN, t3 > - REG_S t3, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) > > /* Save Guest and Restore Host HSTATUS */ > - REG_L t4, (KVM_ARCH_HOST_HSTATUS)(a0) > csrrw t4, CSR_HSTATUS, t4 > - REG_S t4, (KVM_ARCH_GUEST_HSTATUS)(a0) > > /* Save Guest and Restore Host SSTATUS */ > - REG_L t5, (KVM_ARCH_HOST_SSTATUS)(a0) > csrrw t5, CSR_SSTATUS, t5 > + > + /* Store Guest CSR values */ > + REG_S t0, (KVM_ARCH_GUEST_SEPC)(a0) > + REG_S t2, (KVM_ARCH_GUEST_A0)(a0) > + REG_S t3, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) > + REG_S t4, (KVM_ARCH_GUEST_HSTATUS)(a0) > REG_S t5, (KVM_ARCH_GUEST_SSTATUS)(a0) > > /* Restore Host GPRs (except A0 and T0-T6) */ > -- > 2.17.1 >
diff --git a/arch/riscv/kvm/vcpu_switch.S b/arch/riscv/kvm/vcpu_switch.S index 029a28a195c6..d74df8eb4d71 100644 --- a/arch/riscv/kvm/vcpu_switch.S +++ b/arch/riscv/kvm/vcpu_switch.S @@ -41,33 +41,37 @@ ENTRY(__kvm_riscv_switch_to) REG_S s10, (KVM_ARCH_HOST_S10)(a0) REG_S s11, (KVM_ARCH_HOST_S11)(a0) - /* Save Host and Restore Guest SSTATUS */ + /* Load Guest CSR values */ REG_L t0, (KVM_ARCH_GUEST_SSTATUS)(a0) + REG_L t1, (KVM_ARCH_GUEST_HSTATUS)(a0) + REG_L t2, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) + la t4, __kvm_switch_return + REG_L t5, (KVM_ARCH_GUEST_SEPC)(a0) + + /* Save Host and Restore Guest SSTATUS */ csrrw t0, CSR_SSTATUS, t0 - REG_S t0, (KVM_ARCH_HOST_SSTATUS)(a0) /* Save Host and Restore Guest HSTATUS */ - REG_L t1, (KVM_ARCH_GUEST_HSTATUS)(a0) csrrw t1, CSR_HSTATUS, t1 - REG_S t1, (KVM_ARCH_HOST_HSTATUS)(a0) /* Save Host and Restore Guest SCOUNTEREN */ - REG_L t2, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) csrrw t2, CSR_SCOUNTEREN, t2 - REG_S t2, (KVM_ARCH_HOST_SCOUNTEREN)(a0) - - /* Save Host SSCRATCH and change it to struct kvm_vcpu_arch pointer */ - csrrw t3, CSR_SSCRATCH, a0 - REG_S t3, (KVM_ARCH_HOST_SSCRATCH)(a0) /* Save Host STVEC and change it to return path */ - la t4, __kvm_switch_return csrrw t4, CSR_STVEC, t4 - REG_S t4, (KVM_ARCH_HOST_STVEC)(a0) + + /* Save Host SSCRATCH and change it to struct kvm_vcpu_arch pointer */ + csrrw t3, CSR_SSCRATCH, a0 /* Restore Guest SEPC */ - REG_L t0, (KVM_ARCH_GUEST_SEPC)(a0) - csrw CSR_SEPC, t0 + csrw CSR_SEPC, t5 + + /* Store Host CSR values */ + REG_S t0, (KVM_ARCH_HOST_SSTATUS)(a0) + REG_S t1, (KVM_ARCH_HOST_HSTATUS)(a0) + REG_S t2, (KVM_ARCH_HOST_SCOUNTEREN)(a0) + REG_S t3, (KVM_ARCH_HOST_SSCRATCH)(a0) + REG_S t4, (KVM_ARCH_HOST_STVEC)(a0) /* Restore Guest GPRs (except A0) */ REG_L ra, (KVM_ARCH_GUEST_RA)(a0) @@ -145,32 +149,36 @@ __kvm_switch_return: REG_S t5, (KVM_ARCH_GUEST_T5)(a0) REG_S t6, (KVM_ARCH_GUEST_T6)(a0) + /* Load Host CSR values */ + REG_L t1, (KVM_ARCH_HOST_STVEC)(a0) + REG_L t2, (KVM_ARCH_HOST_SSCRATCH)(a0) + REG_L t3, (KVM_ARCH_HOST_SCOUNTEREN)(a0) + REG_L t4, (KVM_ARCH_HOST_HSTATUS)(a0) + REG_L t5, (KVM_ARCH_HOST_SSTATUS)(a0) + /* Save Guest SEPC */ csrr t0, CSR_SEPC - REG_S t0, (KVM_ARCH_GUEST_SEPC)(a0) - - /* Restore Host STVEC */ - REG_L t1, (KVM_ARCH_HOST_STVEC)(a0) - csrw CSR_STVEC, t1 /* Save Guest A0 and Restore Host SSCRATCH */ - REG_L t2, (KVM_ARCH_HOST_SSCRATCH)(a0) csrrw t2, CSR_SSCRATCH, t2 - REG_S t2, (KVM_ARCH_GUEST_A0)(a0) + + /* Restore Host STVEC */ + csrw CSR_STVEC, t1 /* Save Guest and Restore Host SCOUNTEREN */ - REG_L t3, (KVM_ARCH_HOST_SCOUNTEREN)(a0) csrrw t3, CSR_SCOUNTEREN, t3 - REG_S t3, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) /* Save Guest and Restore Host HSTATUS */ - REG_L t4, (KVM_ARCH_HOST_HSTATUS)(a0) csrrw t4, CSR_HSTATUS, t4 - REG_S t4, (KVM_ARCH_GUEST_HSTATUS)(a0) /* Save Guest and Restore Host SSTATUS */ - REG_L t5, (KVM_ARCH_HOST_SSTATUS)(a0) csrrw t5, CSR_SSTATUS, t5 + + /* Store Guest CSR values */ + REG_S t0, (KVM_ARCH_GUEST_SEPC)(a0) + REG_S t2, (KVM_ARCH_GUEST_A0)(a0) + REG_S t3, (KVM_ARCH_GUEST_SCOUNTEREN)(a0) + REG_S t4, (KVM_ARCH_GUEST_HSTATUS)(a0) REG_S t5, (KVM_ARCH_GUEST_SSTATUS)(a0) /* Restore Host GPRs (except A0 and T0-T6) */
Kernel uses __kvm_riscv_switch_to() and __kvm_switch_return() to switch the context of host kernel and guest kernel. Several CSRs belonging to the context will be read and written during the context switch. To ensure atomic read-modify-write control of CSR and ordering of CSR accesses, some hardware blocks flush the pipeline when writing a CSR. In this circumstance, grouping CSR executions together as much as possible can reduce the performance impact of the pipeline. Therefore, this commit reorders the CSR instructions to enhance the context switch performance.. Signed-off-by: Vincent Chen <vincent.chen@sifive.com> Suggested-by: Hsinyi Lee <hsinyi.lee@sifive.com> Suggested-by: Fu-Ching Yang <fu-ching.yang@sifive.com> --- arch/riscv/kvm/vcpu_switch.S | 60 ++++++++++++++++++++---------------- 1 file changed, 34 insertions(+), 26 deletions(-)