Re: [RFC PATCH] hw/sparc/sun4m: Use memdev backend for main RAM

[Mark Cave-Ayland]

On 20/05/2020 11:07, Igor Mammedow wrote:

> On Thu, 14 May 2020 14:13:11 +0200
> Philippe Mathieu-Daudé <address@hidden> wrote:
> 
>> On 5/14/20 12:09 PM, Igor Mammedov wrote:
>>> On Sun, 10 May 2020 13:35:05 +0200
>>> Philippe Mathieu-Daudé <address@hidden> wrote:
>>>   
>>>> Since commit 82b911aaff3, machine_run_board_init() checks for
>>>> ram_memdev_id and consume it. As TYPE_SUN4M_MEMORY is no more
>>>> needed, replace it by the generic memdev allocated MemoryRegion
>>>> and remove it. This completes commit b2554752b1da7c8f.  
>>>
>>> I don't get justification here.
>>> You are removing 'frontend' device model that has little/nothing
>>> to do with how backend is instantiated.
>>>
>>> TYPE_SUN4M_MEMORY is analog to pc-dimm, only for builtin RAM
>>> (not really useful but could serve as an example).  
>>
>> I have no idea about the benefits of using memory frontend/backend
>> with emulation. Is there documentation and examples? I'm seeing this
>> code as a complicated way of doing a simple thing, but I guess I'm
>> missing the big picture here.
> 
> Examples are pc-dimm and nv-dimm which thanks to separation easily
> reusable across pc/spapr/virt-arm.
> 
> Having frontend is also useful for mgmt purposes, where HMP/QMP
> just has to enumerate all RAM devices, otherwise boards would have
> to provide a callback to describe their custom mappings.
> 
> But for embed boards with a single blob of RAM nailed down,
> having frontend is probably overkill (at least ATM).
> So I'm fine with this patch /with amended commit message/

I don't feel too strongly about this, however if there is a standard way of 
doing
things then I would prefer to do that since it makes it easier for me as a 
maintainer
if there are existing (and up-to-date) examples that I can use as a reference.

Presumably it also means that if HMP/QMP commands are added in relation to 
machine
RAM then the SPARC machines would get that new functionality automatically...


ATB,

Mark.