RISC-V: KVM: Refine __kvm_riscv_switch_to() implementation

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>
Signed-off-by: Anup Patel <anup@brainfault.org>

diff --git a/arch/riscv/kvm/vcpu_switch.S b/arch/riscv/kvm/vcpu_switch.S
index 029a28a195c69c08f9cc0c598a3b743905ee8ac3..d74df8eb4d71a56faed0f4a7f4303bef88e6e399 100644 (file)
--- 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) */