Re: KVM: s390: Move two error code assignments in kvm_vm_ioctl_get_dirty_log()

SF Markus Elfring
2017-01-24T12:10:26Z

>> Would you like to check run time consequences
>> for the shown error code settings once more?
>
> Sure, lets for now ignore the fact that the performance of an error path
> does not matter most of the time.

I am concerned that extra error code settings within the “success path”
could influence the run time behaviour in unwanted ways.

> After tree building and optimization your change should not matter at all
> regarding performance for a decent compiler.

I find your optimism interesting.

> The compiler can and will do much more complex transformations than this.

This technology is often fine.

> Since you have send several patches that trigger compile time warnings or
> errors, let me do this exercise for you and let us check what your patch
> changes in terms of run time consequences.
>
>
> $ git checkout v4.10-rc4
> HEAD is now at 49def18... Linux 4.10-rc4
>
> $ make arch/s390/kvm/kvm-s390.o
> [..]

Thanks for your build demonstration.

> $ objdump -d arch/s390/kvm/kvm-s390.o | md5sum
> 55c1e081f55cef90b3ffcc06a13721c1 -
>
> $ git am ~/code/elfring/[PATCH] KVM: s390: Move two error code assignments in kvm_vm_ioctl_get_dirty_log().eml
> Applying: KVM: s390: Move two error code assignments in kvm_vm_ioctl_get_dirty_log()
>
> $ make arch/s390/kvm/kvm-s390.o
> [..]
>
> $ objdump -d arch/s390/kvm/kvm-s390.o | md5sum
> 55c1e081f55cef90b3ffcc06a13721c1 -
>
> As you can see the binary is identical,

The hashes became the same with the selected tool.

> so I can make an educated guess, that there is no performance improvement
> due to your patch.

How much does such a software generation result fit really to expectations?

Should the two shown implementation variants for a function like "kvm_vm_ioctl_get_dirty_log"
usually lead to different object code files?

Regards,
Markus