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Date: Tue, 30 Aug 2016 11:55:34 +0200
From: Christoffer Dall <christoffer.dall@...aro.org>
To: Shanker Donthineni <shankerd@...eaurora.org>
Cc: Marc Zyngier <marc.zyngier@....com>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
kvmarm <kvmarm@...ts.cs.columbia.edu>,
Paolo Bonzini <pbonzini@...hat.com>,
Will Deacon <will.deacon@....com>,
Catalin Marinas <catalin.marinas@....com>,
James Morse <james.morse@....com>
Subject: Re: [PATCH v2] arm64: KVM: Save four instructions in
__guest_enter/exit()
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@...eaurora.org>
This looks reasonable to me:
Reviewed-by: Christoffer Dall <christoffer.dall@...aro.org>
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)
> --
> Qualcomm Datacenter Technologies, Inc. on behalf of the Qualcomm Technologies, Inc.
> Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
>
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