Re: [Qemu-devel] [PATCH v2] arm: add device tree support

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/01/2012 07:55 AM, Alexander Graf wrote:
>
> On 01.02.2012, at 14:52, Anthony Liguori wrote:
>> Fine, but to boot u-boot, the real hardware must set IP to something that's most likely an offset into ROM flash.
>>
>> Why can't we bootstrap semi-hosted mode by having a ROM somewhere that just redirects IP?
>>
>> It doesn't have to be a full blown u-boot.
>
> That would work, yes.
>
>>
>>>
>>> That's why I'm saying things don't work out all that simple with semi-hosted environments. Now you could argue that semi-hosting is a bad thing, but we'll always have to have it. On s390 for example, semi-hosting is how real hardware works. Or at least the parts that are visible to end users. Especially when you model PV machines, you'll have a hard time with fixed reset IPs too.
>>
>> s390 is a special case because "real hardware" is not actually real hardware. It's a VM.
>
> Sure, but how would we model things there? Our model needs to be flexible enough to cope with these oddballs.
>
> In fact, s390 is even more complicated. For DASD boot, the CPU is stalled at first and instead the DASD controller reads some instructions from memory that then bootstrap the bootloader. But IIUC that's only the case for DASD boot. For zfcp boot, you basically get semi-hosting.

Once CPUs are modeled QOM, my expectation is that we'll have something like a 
CPU::halted property.  As part of realize, a CPU would set halted = true and 
that is what would trigger the CPU execution (be it through TCG or KVM).

There is no reason that on s390, the CPU realize function couldn't avoid setting 
halted=true and instead allow another device (with a wider view of the system) 
to perform some additional initialization work and then set the CPU halted 
property to true.

This is all about what causes the system to start running.  Once we move to a 
property realize() model, it gives us a lot more flexibility to work through 
these types of dependency issues.

Regards,

Anthony Liguori

>
> Alex
>
>