* [PATCH] arm64: KVM: Save two instructions in __guest_enter()
@ 2016-08-10 1:15 Shanker Donthineni
2016-08-25 13:31 ` Christoffer Dall
2016-08-29 18:13 ` Christoffer Dall
0 siblings, 2 replies; 5+ messages in thread
From: Shanker Donthineni @ 2016-08-10 1:15 UTC (permalink / raw)
To: Marc Zyngier, Christoffer Dall, linux-kernel, linux-arm-kernel, kvmarm
Cc: Paolo Bonzini, Will Deacon, Catalin Marinas, James Morse,
Shanker Donthineni
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 and not
being used anywhere, the same information is available in tpidr_el2.
Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
---
arch/arm64/kvm/hyp/entry.S | 66 ++++++++++++++++++++++------------------------
1 file changed, 32 insertions(+), 34 deletions(-)
diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index ce9e5e5..d2e09a1 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -55,37 +55,32 @@
*/
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]!
+ // Preserve 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 x19-x29, lr
+ restore_callee_saved_regs x18
- // 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)]
-
- // x19-x29, lr
- restore_callee_saved_regs x1
-
- // Last bits of the 64bit state
- ldp x0, x1, [sp], #16
+ // Restore guest regs x0-x18
+ 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)]
+ ldr x18, [x18, #CPU_XREG_OFFSET(18)]
// Do not touch any register after this!
eret
@@ -100,6 +95,16 @@ ENTRY(__guest_exit)
add x2, x0, #VCPU_CONTEXT
+ // Store the guest regs x19-x29, lr
+ save_callee_saved_regs x2
+
+ // Retrieve the guest regs x0-x3 from the stack
+ ldp x21, x22, [sp], #16 // x2, x3
+ ldp x19, x20, [sp], #16 // x0, x1
+
+ // Store the guest regs x0-x18
+ stp x19, x20, [x2, #CPU_XREG_OFFSET(0)]
+ stp x21, x22, [x2, #CPU_XREG_OFFSET(2)]
stp x4, x5, [x2, #CPU_XREG_OFFSET(4)]
stp x6, x7, [x2, #CPU_XREG_OFFSET(6)]
stp x8, x9, [x2, #CPU_XREG_OFFSET(8)]
@@ -109,20 +114,13 @@ ENTRY(__guest_exit)
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
+ // Restore the host_ctxt from the stack
+ ldr x2, [sp], #16
- stp x4, x5, [x2, #CPU_XREG_OFFSET(0)]
- stp x6, x7, [x2, #CPU_XREG_OFFSET(2)]
-
- save_callee_saved_regs x2
-
- // 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
+ // Preserving return code (x1)
mov x0, x1
ret
ENDPROC(__guest_exit)
--
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.
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()
2016-08-10 1:15 [PATCH] arm64: KVM: Save two instructions in __guest_enter() Shanker Donthineni
@ 2016-08-25 13:31 ` Christoffer Dall
2016-08-29 15:22 ` Shanker Donthineni
2016-08-29 18:13 ` Christoffer Dall
1 sibling, 1 reply; 5+ messages in thread
From: Christoffer Dall @ 2016-08-25 13:31 UTC (permalink / raw)
To: Shanker Donthineni
Cc: Marc Zyngier, linux-kernel, linux-arm-kernel, kvmarm,
Paolo Bonzini, Will Deacon, Catalin Marinas, James Morse
Hi Shanker,
On Tue, Aug 09, 2016 at 08:15:36PM -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 and not
> being used anywhere, the same information is available in tpidr_el2.
Does this have any measureable benefit?
Thanks,
-Christoffer
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()
2016-08-25 13:31 ` Christoffer Dall
@ 2016-08-29 15:22 ` Shanker Donthineni
2016-08-29 18:15 ` Christoffer Dall
0 siblings, 1 reply; 5+ messages in thread
From: Shanker Donthineni @ 2016-08-29 15:22 UTC (permalink / raw)
To: Christoffer Dall
Cc: Marc Zyngier, linux-kernel, linux-arm-kernel, kvmarm,
Paolo Bonzini, Will Deacon, Catalin Marinas, James Morse
Hi Christoffer,
This is change may not provide the measurable performance improvement,
but still we can
save a few cpu cycles on vCPU context switch and also improves the code
readability.
On 08/25/2016 08:31 AM, Christoffer Dall wrote:
> Hi Shanker,
>
> On Tue, Aug 09, 2016 at 08:15:36PM -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 and not
>> being used anywhere, the same information is available in tpidr_el2.
> Does this have any measureable benefit?
>
> Thanks,
> -Christoffer
--
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()
2016-08-29 15:22 ` Shanker Donthineni
@ 2016-08-29 18:15 ` Christoffer Dall
0 siblings, 0 replies; 5+ messages in thread
From: Christoffer Dall @ 2016-08-29 18:15 UTC (permalink / raw)
To: Shanker Donthineni
Cc: Marc Zyngier, linux-kernel, linux-arm-kernel, kvmarm,
Paolo Bonzini, Will Deacon, Catalin Marinas, James Morse
On Mon, Aug 29, 2016 at 10:22:06AM -0500, Shanker Donthineni wrote:
> Hi Christoffer,
>
> This is change may not provide the measurable performance
> improvement, but still we can
> save a few cpu cycles on vCPU context switch and also improves the
> code readability.
I'm not so convinced about saving CPU cycles, but now when I've reviewed
your patch I think it has potential to slightly improve the code. See
my review of your patch.
-Christoffer
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()
2016-08-10 1:15 [PATCH] arm64: KVM: Save two instructions in __guest_enter() Shanker Donthineni
2016-08-25 13:31 ` Christoffer Dall
@ 2016-08-29 18:13 ` Christoffer Dall
1 sibling, 0 replies; 5+ messages in thread
From: Christoffer Dall @ 2016-08-29 18:13 UTC (permalink / raw)
To: Shanker Donthineni
Cc: Marc Zyngier, linux-kernel, linux-arm-kernel, kvmarm,
Paolo Bonzini, Will Deacon, Catalin Marinas, James Morse
On Tue, Aug 09, 2016 at 08:15:36PM -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 and not
> being used anywhere, the same information is available in tpidr_el2.
>
> Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
> ---
> arch/arm64/kvm/hyp/entry.S | 66 ++++++++++++++++++++++------------------------
> 1 file changed, 32 insertions(+), 34 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index ce9e5e5..d2e09a1 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -55,37 +55,32 @@
> */
> 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]!
> + // Preserve 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 x19-x29, lr
> + restore_callee_saved_regs x18
couldn't moving this load here be bad for prefetching?
>
> - // 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)]
> -
> - // x19-x29, lr
> - restore_callee_saved_regs x1
> -
> - // Last bits of the 64bit state
> - ldp x0, x1, [sp], #16
> + // Restore guest regs x0-x18
> + 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)]
> + ldr x18, [x18, #CPU_XREG_OFFSET(18)]
>
> // Do not touch any register after this!
> eret
> @@ -100,6 +95,16 @@ ENTRY(__guest_exit)
>
> add x2, x0, #VCPU_CONTEXT
>
> + // Store the guest regs x19-x29, lr
> + save_callee_saved_regs x2
same question here (although with a different weight as we were already
'jumping back' with the memory address in our store sequence.
If this is a real concern, a better approach would be to override x0
with the vcpu context pointer, do two pairs of load/stores using x2,x3
for the vcpu x0-x3, and then proceed with the rest of the registers.
> +
> + // Retrieve the guest regs x0-x3 from the stack
> + ldp x21, x22, [sp], #16 // x2, x3
> + ldp x19, x20, [sp], #16 // x0, x1
> +
> + // Store the guest regs x0-x18
> + stp x19, x20, [x2, #CPU_XREG_OFFSET(0)]
> + stp x21, x22, [x2, #CPU_XREG_OFFSET(2)]
> stp x4, x5, [x2, #CPU_XREG_OFFSET(4)]
> stp x6, x7, [x2, #CPU_XREG_OFFSET(6)]
> stp x8, x9, [x2, #CPU_XREG_OFFSET(8)]
> @@ -109,20 +114,13 @@ ENTRY(__guest_exit)
> 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
> + // Restore the host_ctxt from the stack
> + ldr x2, [sp], #16
>
> - stp x4, x5, [x2, #CPU_XREG_OFFSET(0)]
> - stp x6, x7, [x2, #CPU_XREG_OFFSET(2)]
> -
> - save_callee_saved_regs x2
> -
> - // 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
>
> + // Preserving return code (x1)
nit: preserve is a strange word to choose to describe what you do here.
if you want to do what I suggested above, you could change the two
callers to return the return code in x0, and the vcpu pointer in x1 and
then you can save this instruction as well.
> mov x0, x1
> ret
> ENDPROC(__guest_exit)
> --
Thanks,
-Christoffer
^ permalink raw reply [flat|nested] 5+ messages in thread
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2016-08-10 1:15 [PATCH] arm64: KVM: Save two instructions in __guest_enter() Shanker Donthineni
2016-08-25 13:31 ` Christoffer Dall
2016-08-29 15:22 ` Shanker Donthineni
2016-08-29 18:15 ` Christoffer Dall
2016-08-29 18:13 ` Christoffer Dall
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