Re: [PATCH v2] arm64: KVM: Save four instructions in __guest_enter/exit()

[Christoffer Dall]

On Mon, Aug 29, 2016 at 10:51:14PM -0500, Shanker Donthineni wrote:
> We are doing an unnecessary stack push/pop operation when restoring
> the guest registers x0-x18 in __guest_enter(). This patch saves the
> two instructions by using x18 as a base register. No need to store
> the vcpu context pointer in stack because it is redundant, the same
> information is available in tpidr_el2. The function __guest_exit()
> prototype is simplified and caller pushes the regs x0-x1 to stack
> instead of regs x0-x3.
> 
> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>

This looks reasonable to me:

Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>

Unless Marc has any insight into this having a negative effect on ARM
CPUs, I'll go ahead an merge this.

-Christoffer

> ---
> Changes since v1:
>   Incorporated Cristoffer suggestions.
>   __guest_exit prototype is changed to 'void __guest_exit(u64 reason, struct kvm_vcpu *vcpu)'.
> 
>  arch/arm64/kvm/hyp/entry.S     | 101 +++++++++++++++++++++--------------------
>  arch/arm64/kvm/hyp/hyp-entry.S |  11 +++--
>  2 files changed, 57 insertions(+), 55 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index ce9e5e5..f70489a 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -55,75 +55,76 @@
>   */
>  ENTRY(__guest_enter)
>  	// x0: vcpu
> -	// x1: host/guest context
> -	// x2-x18: clobbered by macros
> +	// x1: host context
> +	// x2-x17: clobbered by macros
> +	// x18: guest context
>  
>  	// Store the host regs
>  	save_callee_saved_regs x1
>  
> -	// Preserve vcpu & host_ctxt for use at exit time
> -	stp	x0, x1, [sp, #-16]!
> +	// Store the host_ctxt for use at exit time
> +	str	x1, [sp, #-16]!
>  
> -	add	x1, x0, #VCPU_CONTEXT
> +	add	x18, x0, #VCPU_CONTEXT
>  
> -	// Prepare x0-x1 for later restore by pushing them onto the stack
> -	ldp	x2, x3, [x1, #CPU_XREG_OFFSET(0)]
> -	stp	x2, x3, [sp, #-16]!
> +	// Restore guest regs x0-x17
> +	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
> +	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
> +	ldp	x4, x5,   [x18, #CPU_XREG_OFFSET(4)]
> +	ldp	x6, x7,   [x18, #CPU_XREG_OFFSET(6)]
> +	ldp	x8, x9,   [x18, #CPU_XREG_OFFSET(8)]
> +	ldp	x10, x11, [x18, #CPU_XREG_OFFSET(10)]
> +	ldp	x12, x13, [x18, #CPU_XREG_OFFSET(12)]
> +	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
> +	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
>  
> -	// x2-x18
> -	ldp	x2, x3,   [x1, #CPU_XREG_OFFSET(2)]
> -	ldp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
> -	ldp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> -	ldp	x8, x9,   [x1, #CPU_XREG_OFFSET(8)]
> -	ldp	x10, x11, [x1, #CPU_XREG_OFFSET(10)]
> -	ldp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
> -	ldp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
> -	ldp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> -	ldr	x18,      [x1, #CPU_XREG_OFFSET(18)]
> +	// Restore guest regs x19-x29, lr
> +	restore_callee_saved_regs x18
>  
> -	// x19-x29, lr
> -	restore_callee_saved_regs x1
> -
> -	// Last bits of the 64bit state
> -	ldp	x0, x1, [sp], #16
> +	// Restore guest reg x18
> +	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
>  
>  	// Do not touch any register after this!
>  	eret
>  ENDPROC(__guest_enter)
>  
> +/*
> + * void __guest_exit(u64 exit_reason, struct kvm_vcpu *vcpu);
> + */
>  ENTRY(__guest_exit)
> -	// x0: vcpu
> -	// x1: return code
> -	// x2-x3: free
> -	// x4-x29,lr: vcpu regs
> -	// vcpu x0-x3 on the stack
> -
> -	add	x2, x0, #VCPU_CONTEXT
> -
> -	stp	x4, x5,   [x2, #CPU_XREG_OFFSET(4)]
> -	stp	x6, x7,   [x2, #CPU_XREG_OFFSET(6)]
> -	stp	x8, x9,   [x2, #CPU_XREG_OFFSET(8)]
> -	stp	x10, x11, [x2, #CPU_XREG_OFFSET(10)]
> -	stp	x12, x13, [x2, #CPU_XREG_OFFSET(12)]
> -	stp	x14, x15, [x2, #CPU_XREG_OFFSET(14)]
> -	stp	x16, x17, [x2, #CPU_XREG_OFFSET(16)]
> -	str	x18,      [x2, #CPU_XREG_OFFSET(18)]
> -
> -	ldp	x6, x7, [sp], #16	// x2, x3
> -	ldp	x4, x5, [sp], #16	// x0, x1
> -
> -	stp	x4, x5, [x2, #CPU_XREG_OFFSET(0)]
> -	stp	x6, x7, [x2, #CPU_XREG_OFFSET(2)]
> +	// x0: return code
> +	// x1: vcpu
> +	// x2-x29,lr: vcpu regs
> +	// vcpu x0-x1 on the stack
> +
> +	add	x1, x1, #VCPU_CONTEXT
> +
> +	// Store the guest regs x2 and x3
> +	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(2)]
> +
> +	// Retrieve the guest regs x0-x1 from the stack
> +	ldp	x2, x3, [sp], #16	// x0, x1
> +
> +	// Store the guest regs x0-x1 and x4-x18
> +	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
> +	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
> +	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> +	stp	x8, x9,   [x1, #CPU_XREG_OFFSET(8)]
> +	stp	x10, x11, [x1, #CPU_XREG_OFFSET(10)]
> +	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
> +	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
> +	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> +	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
> +
> +	// Store the guest regs x19-x29, lr
> +	save_callee_saved_regs x1
>  
> -	save_callee_saved_regs x2
> +	// Restore the host_ctxt from the stack
> +	ldr	x2, [sp], #16
>  
> -	// Restore vcpu & host_ctxt from the stack
> -	// (preserving return code in x1)
> -	ldp	x0, x2, [sp], #16
>  	// Now restore the host regs
>  	restore_callee_saved_regs x2
>  
> -	mov	x0, x1
>  	ret
>  ENDPROC(__guest_exit)
>  
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index f6d9694..06e8b3b 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -121,14 +121,15 @@ el1_trap:
>  	cmp	x2, #ESR_ELx_EC_FP_ASIMD
>  	b.eq	__fpsimd_guest_restore
>  
> -	mrs	x0, tpidr_el2
> -	mov	x1, #ARM_EXCEPTION_TRAP
> +	ldp     x2, x3, [sp], #16
> +	mrs	x1, tpidr_el2
> +	mov	x0, #ARM_EXCEPTION_TRAP
>  	b	__guest_exit
>  
>  el1_irq:
> -	save_x0_to_x3
> -	mrs	x0, tpidr_el2
> -	mov	x1, #ARM_EXCEPTION_IRQ
> +	stp     x0, x1, [sp, #-16]!
> +	mrs	x1, tpidr_el2
> +	mov	x0, #ARM_EXCEPTION_IRQ
>  	b	__guest_exit
>  
>  ENTRY(__hyp_do_panic)
> -- 
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