Re: [Qemu-devel] [PATCH 3/3] target-xtensa: xtfpga: support noMMU cores

[Max Filippov]

On Mon, Sep 28, 2015 at 12:28 AM, Peter Crosthwaite
<address@hidden> wrote:
> On Sun, Sep 27, 2015 at 12:01 PM, Max Filippov <address@hidden> wrote:
>> On Sun, Sep 27, 2015 at 9:43 PM, Peter Crosthwaite
>> <address@hidden> wrote:
>>> On Sun, Sep 27, 2015 at 11:13 AM, Max Filippov <address@hidden> wrote:
>>>> On Sun, Sep 27, 2015 at 8:38 PM, Peter Crosthwaite
>>>>>> +    mmu = xtensa_option_enabled(env->config, XTENSA_OPTION_MMU);
>>>>>
>>>>> This looks backwards, the board should be in charge of itself and the
>>>>> CPU config, rather than spying on the CPU setup to rewire the board.
>>>>
>>>> Well, it's an FPGA board and all connections are a part of bitstream.
>>>> It's generated that way, I'm just following the specification here.
>>>>
>>>
>>> OK, but the xtensa-CPU is not the bitstream, this board is. What
>>> exactly is the user interface for switching between MMU and no-MMU?
>>
>> Actually they both are. The user interface is a dropbox in the processor
>> generator software where user chooses memory management option.
>> Once it (and a bunch of other parameters) is chosen the bitstream with
>> CPU and peripherals can be generated.
>>
>>> With the major changes of address layout, the no-MMU variation should
>>> be a set of new boards or a machine level parameterisation (i.e. QOM
>>> property of the machine). It needs to be user-visible as different on
>>> the machine level.
>>
>> Why? The layouts are hard-coded based on MMU presence anyway.
>
> So this really means that MMUness is board level property, not a CPU
> level property.

CPU is primary, because one can generate RTL for CPU only. If a whole
FPGA bitstream is generated, address space layout will be chosen in
accordance with MMU presence in CPU.

> To clarify, can you tell me the QEMU command line difference between
> MMU and noMMU?

There is no difference. You specify -cpu without full MMU -- you get
noMMU address space layout. There are no noMMU cores in the QEMU
mainline ATM, but there are cpu3400, dsp3400, lx106 and de108 in the
xtensa-cores branch of my tree
( https://github.com/OSLL/qemu-xtensa/commits/xtensa-cores ).

-- 
Thanks.
-- Max