Re: [PATCH v3 1/2] arm64: KVM: Optimize arm64 skip 30-50% vfp/simd save/restore on exits

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Jun 24, 2015 at 05:04:11PM -0700, Mario Smarduch wrote:
> This patch only saves and restores FP/SIMD registers on Guest access. To do
> this cptr_el2 FP/SIMD trap is set on Guest entry and later checked on exit.
> lmbench, hackbench show significant improvements, for 30-50% exits FP/SIMD
> context is not saved/restored
> 
> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
> ---
>  arch/arm64/include/asm/kvm_arm.h |    5 ++++-
>  arch/arm64/kvm/hyp.S             |   46 +++++++++++++++++++++++++++++++++++---
>  2 files changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index ac6fafb..7605e09 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -171,10 +171,13 @@
>  #define HSTR_EL2_TTEE	(1 << 16)
>  #define HSTR_EL2_T(x)	(1 << x)
>  
> +/* Hyp Coproccessor Trap Register Shifts */
> +#define CPTR_EL2_TFP_SHIFT 10
> +
>  /* Hyp Coprocessor Trap Register */
>  #define CPTR_EL2_TCPAC	(1 << 31)
>  #define CPTR_EL2_TTA	(1 << 20)
> -#define CPTR_EL2_TFP	(1 << 10)
> +#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
>  
>  /* Hyp Debug Configuration Register bits */
>  #define MDCR_EL2_TDRA		(1 << 11)
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index 5befd01..de0788f 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -673,6 +673,15 @@
>  	tbz	\tmp, #KVM_ARM64_DEBUG_DIRTY_SHIFT, \target
>  .endm
>  
> +/*
> + * Check cptr VFP/SIMD accessed bit, if set VFP/SIMD not accessed by guest.

This comment doesn't really help me understand the function, may I
suggest:

Branch to target if CPTR_EL2.TFP bit is set (VFP/SIMD trapping enabled)

> + */
> +.macro skip_fpsimd_state tmp, target
> +	mrs	\tmp, cptr_el2
> +	tbnz	\tmp, #CPTR_EL2_TFP_SHIFT, \target
> +.endm
> +
> +
>  .macro compute_debug_state target
>  	// Compute debug state: If any of KDE, MDE or KVM_ARM64_DEBUG_DIRTY
>  	// is set, we do a full save/restore cycle and disable trapping.
> @@ -763,6 +772,7 @@
>  	ldr     x2, [x0, #VCPU_HCR_EL2]
>  	msr     hcr_el2, x2
>  	mov	x2, #CPTR_EL2_TTA
> +	orr     x2, x2, #CPTR_EL2_TFP
>  	msr	cptr_el2, x2
>  
>  	mov	x2, #(1 << 15)	// Trap CP15 Cr=15
> @@ -785,7 +795,6 @@
>  .macro deactivate_traps
>  	mov	x2, #HCR_RW
>  	msr	hcr_el2, x2
> -	msr	cptr_el2, xzr
>  	msr	hstr_el2, xzr
>  
>  	mrs	x2, mdcr_el2
> @@ -912,6 +921,28 @@ __restore_fpsimd:
>  	restore_fpsimd
>  	ret
>  
> +switch_to_guest_fpsimd:
> +	push	x4, lr
> +
> +	mrs	x2, cptr_el2
> +	bic	x2, x2, #CPTR_EL2_TFP
> +	msr	cptr_el2, x2
> +
> +	mrs	x0, tpidr_el2
> +
> +	ldr	x2, [x0, #VCPU_HOST_CONTEXT]
> +	kern_hyp_va x2
> +	bl __save_fpsimd
> +
> +	add	x2, x0, #VCPU_CONTEXT
> +	bl __restore_fpsimd
> +
> +	pop	x4, lr
> +	pop	x2, x3
> +	pop	x0, x1
> +
> +	eret
> +
>  /*
>   * u64 __kvm_vcpu_run(struct kvm_vcpu *vcpu);
>   *
> @@ -932,7 +963,6 @@ ENTRY(__kvm_vcpu_run)
>  	kern_hyp_va x2
>  
>  	save_host_regs
> -	bl __save_fpsimd
>  	bl __save_sysregs
>  
>  	compute_debug_state 1f
> @@ -948,7 +978,6 @@ ENTRY(__kvm_vcpu_run)
>  	add	x2, x0, #VCPU_CONTEXT
>  
>  	bl __restore_sysregs
> -	bl __restore_fpsimd
>  
>  	skip_debug_state x3, 1f
>  	bl	__restore_debug
> @@ -967,7 +996,9 @@ __kvm_vcpu_return:
>  	add	x2, x0, #VCPU_CONTEXT
>  
>  	save_guest_regs
> +	skip_fpsimd_state x3, 1f
>  	bl __save_fpsimd
> +1:
>  	bl __save_sysregs
>  
>  	skip_debug_state x3, 1f
> @@ -986,7 +1017,11 @@ __kvm_vcpu_return:
>  	kern_hyp_va x2
>  
>  	bl __restore_sysregs
> +	skip_fpsimd_state x3, 1f
>  	bl __restore_fpsimd
> +1:
> +	/* Clear FPSIMD and Trace trapping */
> +	msr     cptr_el2, xzr

why not simply move the deactivate_traps down here instead?

>  
>  	skip_debug_state x3, 1f
>  	// Clear the dirty flag for the next run, as all the state has
> @@ -1201,6 +1236,11 @@ el1_trap:
>  	 * x1: ESR
>  	 * x2: ESR_EC
>  	 */
> +
> +	/* Guest accessed VFP/SIMD registers, save host, restore Guest */
> +	cmp	x2, #ESR_ELx_EC_FP_ASIMD
> +	b.eq	switch_to_guest_fpsimd
> +
>  	cmp	x2, #ESR_ELx_EC_DABT_LOW
>  	mov	x0, #ESR_ELx_EC_IABT_LOW
>  	ccmp	x2, x0, #4, ne
> -- 
> 1.7.9.5
> 

Otherwise looks good,
-Christoffer