[PATCH] arm64: KVM: reduce guest fpsimd trap

[marc.zyngier@arm.com (Marc Zyngier)]

[+Dave]

Hi Nianyao,

On 16/05/18 10:08, Tangnianyao (ICT) wrote:
> Add last_fpsimd_trap to notify if guest last exit reason is handling fpsimd. If guest is using fpsimd frequently, save host's fpsimd state and restore guest's fpsimd state and deactive fpsimd trap before returning to guest. It can avoid guest fpsimd trap soon to improve performance.
> 
> Signed-off-by: Nianyao Tang <tangnianyao@huawei.com>
> ---
>  arch/arm64/kernel/asm-offsets.c |  1 +
>  arch/arm64/kvm/hyp/entry.S      |  5 +++++
>  arch/arm64/kvm/hyp/switch.c     | 38 ++++++++++++++++++++++++++++++++++++--
>  include/linux/kvm_host.h        |  1 +
>  4 files changed, 43 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c index 5bdda65..35a9c5c 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -136,6 +136,7 @@ int main(void)
>  #ifdef CONFIG_KVM_ARM_HOST
>    DEFINE(VCPU_CONTEXT,		offsetof(struct kvm_vcpu, arch.ctxt));
>    DEFINE(VCPU_FAULT_DISR,	offsetof(struct kvm_vcpu, arch.fault.disr_el1));
> +  DEFINE(VCPU_LAST_FPSIMD_TRAP, offsetof(struct kvm_vcpu, 
> + last_fpsimd_trap));
>    DEFINE(CPU_GP_REGS,		offsetof(struct kvm_cpu_context, gp_regs));
>    DEFINE(CPU_USER_PT_REGS,	offsetof(struct kvm_regs, regs));
>    DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S index e41a161..956e042 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -197,6 +197,11 @@ alternative_endif
>  	add	x0, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
>  	bl	__fpsimd_restore_state
>  
> +	// Mark guest using fpsimd now
> +	ldr x0, [x3, #VCPU_LAST_FPSIMD_TRAP]
> +	add x0, x0, #1
> +	str x0, [x3, #VCPU_LAST_FPSIMD_TRAP]
> +
>  	// Skip restoring fpexc32 for AArch64 guests
>  	mrs	x1, hcr_el2
>  	tbnz	x1, #HCR_RW_SHIFT, 1f
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c index d964523..86eea1b 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -92,7 +92,13 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
>  
>  	val = read_sysreg(cpacr_el1);
>  	val |= CPACR_EL1_TTA;
> -	val &= ~(CPACR_EL1_FPEN | CPACR_EL1_ZEN);
> +	val &= ~CPACR_EL1_ZEN;
> +
> +	if (vcpu->last_fpsimd_trap)
> +		val |= CPACR_EL1_FPEN;
> +	else
> +		val &= ~CPACR_EL1_FPEN;
> +
>  	write_sysreg(val, cpacr_el1);
>  
>  	write_sysreg(kvm_get_hyp_vector(), vbar_el1); @@ -105,7 +111,13 @@ static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
>  	__activate_traps_common(vcpu);
>  
>  	val = CPTR_EL2_DEFAULT;
> -	val |= CPTR_EL2_TTA | CPTR_EL2_TFP | CPTR_EL2_TZ;
> +	val |= CPTR_EL2_TTA | CPTR_EL2_TZ;
> +
> +	if (vcpu->last_fpsimd_trap)
> +		val &= ~CPTR_EL2_TFP;
> +	else
> +		val |= CPTR_EL2_TFP;
> +
>  	write_sysreg(val, cptr_el2);
>  }
>  
> @@ -406,6 +418,17 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
>  	__activate_traps(vcpu);
>  	__activate_vm(vcpu->kvm);
>  
> +	/*
> +	 * If guest last trap to host for handling fpsimd, last_fpsimd_trap
> +	 * is set. Restore guest's fpsimd state and deactivate fpsimd trap
> +	 * to avoid guest traping soon.
> +	 */
> +	if (vcpu->last_fpsimd_trap) {
> +		__fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> +		__fpsimd_restore_state(&guest_ctxt->gp_regs.fp_regs);
> +		vcpu->last_fpsimd_trap = 0;
> +	}
> +
>  	sysreg_restore_guest_state_vhe(guest_ctxt);
>  	__debug_switch_to_guest(vcpu);
>  
> @@ -454,6 +477,17 @@ int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
>  	__activate_traps(vcpu);
>  	__activate_vm(kern_hyp_va(vcpu->kvm));
>  
> +	/*
> +	 * If guest last trap to host for handling fpsimd, last_fpsimd_trap
> +	 * is set. Restore guest's fpsimd state and deactivate fpsimd trap
> +	 * to avoid guest traping soon.
> +	 */
> +	if (vcpu->last_fpsimd_trap) {
> +		__fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> +		__fpsimd_restore_state(&guest_ctxt->gp_regs.fp_regs);
> +		vcpu->last_fpsimd_trap = 0;
> +	}
> +
>  	__hyp_vgic_restore_state(vcpu);
>  	__timer_enable_traps(vcpu);
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 6930c63..46bdf0d 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -274,6 +274,7 @@ struct kvm_vcpu {
>  	bool preempted;
>  	struct kvm_vcpu_arch arch;
>  	struct dentry *debugfs_dentry;
> +	unsigned int last_fpsimd_trap;
>  };
>  
>  static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
> --
> 2.7.4
> 

This doesn't seem to be the 100% correct. I can't see how this works
when being preempted, for example... I suggest you look at Dave Martin's
series[1], which does this correctly.

Thanks,

	M.

[1] https://www.mail-archive.com/kvmarm at lists.cs.columbia.edu/msg17563.html
-- 
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