Re: [Xen-devel] [PATCH] xen/arm: io: add function swap_mmio_handler()

[chenbaodong <chenbaodong@mxnavi.com>]

On 6/12/19 17:08, Julien Grall wrote:
> Hi,
>
> On 6/12/19 6:42 AM, Baodong Chen wrote:
>> Swap function can be used when calling sort().
>> or else, the default swap function generic_swap() is used,
>> which is a little inefficient.
>
> I am not entirely convince this will be more efficient. mmio_handler 
> does not fit in 64 bit, so the compiler may decide to do either 
> multiple load or replace with a memcpy.

Hello Julien,

I have checked the disassemble result,

and IIUC generic_swap has a loop so it should be a little inefficient. 
I'm not expert about hardware, please correct me if i'm wrong.

000000000022ee88 <generic_swap>:
   22ee88:       d2800003        mov     x3, #0x0                        
// #0
   22ee8c:       d503201f        nop
   22ee90:       38636825        ldrb    w5, [x1, x3]
   22ee94:       38636804        ldrb    w4, [x0, x3]
   22ee98:       38236805        strb    w5, [x0, x3]
   22ee9c:       38236824        strb    w4, [x1, x3]
   22eea0:       91000463        add     x3, x3, #0x1
   22eea4:       4b030044        sub     w4, w2, w3
   22eea8:       7100009f        cmp     w4, #0x0
   22eeac:       54ffff2c        b.gt    22ee90 <generic_swap+0x8>
   22eeb0:       d65f03c0        ret
   22eeb4:       d503201f        nop


0000000000242db8 <swap_mmio_handler>:
   242db8:       a9400c22        ldp     x2, x3, [x1]
   242dbc:       d10083ff        sub     sp, sp, #0x20
   242dc0:       a9401404        ldp     x4, x5, [x0]
   242dc4:       a9000c02        stp     x2, x3, [x0]
   242dc8:       a9410c02        ldp     x2, x3, [x0, #16]
   242dcc:       a9411c26        ldp     x6, x7, [x1, #16]
   242dd0:       a9011c06        stp     x6, x7, [x0, #16]
   242dd4:       a9001424        stp     x4, x5, [x1]
   242dd8:       a9010c22        stp     x2, x3, [x1, #16]
   242ddc:       910083ff        add     sp, sp, #0x20
   242de0:       d65f03c0        ret
   242de4:       d503201f        nop

>
> So at best this feels some micro-optimization. But then, this is only 
> call a limited number of time at each domain build. Is it really worth 
> it?

It's not hot path here.

Not sure about worth.

Personally  i will try my best to do things well according to my 
understanding.

>
> On a side note, I have noticed you are sending a lot of 
> optimization/clean-up patch. What is your end goal here?

My goal is to understand how xen works well.

>
> If it is to improve the performance, then there are much bigger fish 
> to fry within Xen code base. I am happy to point some of them based on 
> where you are looking to improve.

Surly i want to improve performance.

Features like Fast Startup ( I learned from xen summit 2018, samsung 
automotive presentation).

But currently i don't understand xen well, only a few weeks experience.

I'm afraid can't catch big fish.

>
> Cheers,
>

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