Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> > Only I can guess for that reason is the reduction of some function calls
> > by inlining some functions.
>
> Yes once at a time cfu was inline too and just checked for the right
> sizes and the used g*u, but it got lost in the "icache over everything else" mania which is unfortunately en vogue for quite some time in kernel land (aka
> measuring patches only based on their impact on the .text
> size, not the actual performance)
>
> But you might getter better gains by fixing this general
> c*u() regression.
>
What I mentioned was about not only cfu but 3 functions.
Originally, we were doing the following function calls:
walk_addr_generic() ---1
kvm_read_guest_page_mmu() ---2
kvm_read_guest_page() ---3
copy_from_user() ---4
And now, with my patch already applied, we are not using
generic kvm_read_guest_page_mmu() and some address
calculations are done in walk_addr_generic():
walk_addr_generic() ---1'
get_user() ---2'
The length is passed in from walk_addr_generic().
Do you think the following case would not differ so much
from (1' 2') ?
walk_addr_generic() ---1''
copy_from_user() ---2''
This can satisfy your "assuming you pass in a constant length"
and almost same as get_user() ?
Takuya
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