[PATCH v2] RISC-V: KVM: Refine __kvm_riscv_switch_to() implementation

[Anup Patel]

On Wed, Feb 23, 2022 at 6:43 AM Vincent Chen <vincent.chen at 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 at sifive.com>
> Suggested-by: Hsinyi Lee <hsinyi.lee at sifive.com>
> Suggested-by: Fu-Ching Yang <fu-ching.yang at 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
>