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Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()

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  Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter() Christoffer Dall <christoffer.dall@linaro.org> - 2016-08-25 15:40 +0200
    Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter() Shanker Donthineni <shankerd@codeaurora.org> - 2016-08-29 17:30 +0200
      Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter() Christoffer Dall <christoffer.dall@linaro.org> - 2016-08-29 20:20 +0200

#1470152 — Re: [PATCH] arm64: KVM: Save two instructions in __guest_enter()

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

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#1471920

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.

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#1472039

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

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