Re: [PATCH v2] arm64: KVM: Save four instructions in __guest_enter/exit()
From: Marc Zyngier
Date: Tue Aug 30 2016 - 06:54:41 EST
On 30/08/16 10:55, Christoffer Dall wrote:
> 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.
I've given it a go on Seattle, and couldn't observe any difference with
the original code, which is pretty good news!
I have some comments below, though:
>
> -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);
>> + */
I'm not sure this comment makes much sense as it stands. This is not a C
function by any stretch of the imagination, but the continuation of
__guest_enter. The calling convention is not the C one at all (see how
the stack is involved), and caller-saved registers are going to be
clobbered.
>> 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
So the save_x0_to_x3 macro now only has one single user (and so does
restore_x0_to_x3). Should we consider inline it?
>> - 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.
>>
>
Thanks,
M.
--
Jazz is not dead. It just smells funny...