Re: Xen ARM small task (WAS: Re: [Xen Question])

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 11/11/2016 13:55, Stefano Stabellini wrote:
> On Fri, 11 Nov 2016, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 11/11/2016 02:24, Stefano Stabellini wrote:
>>> On Thu, 10 Nov 2016, Julien Grall wrote:
>>>> (CC Stefano and change the title)
>>>>
>>>> Hello,
>>>>
>>>> On 10/11/16 12:13, casionwoo wrote:
>>>>> I’m pleased about your reply and you have a lot of code to clean-up.
>>>>>
>>>>> As you mentioned, It’s really huge to digest at once. Thank you for
>>>>> understanding me.
>>>>>
>>>>> And that’s why i need a small fix up and todo list.
>>>>>
>>>>> I feel familiar with ARM and c language and there’s no specific area
>>>>> yet.
>>>>>
>>>>> I think that i can find interesting area with following up the codes.
>>>>>
>>>>> I’m looking forward to being stuck on Xen.
>>>>>
>>>>> Then it would be easier for me to understand about Xen on ARM.
>>>>>
>>>>> Please let me know the TODO and bug-fix lists.
>>>>
>>>> Stefano, before giving a list of code clean-up, do you have any small TODO
>>>> on
>>>> ARM in mind?
>>>
>>> A simple task we talked about recently is to enable the vuart
>>> (xen/arch/arm/vuart.c) for all guests. At the moment it is only emulated
>>> for Dom0, but some guests, in particular BareMetal guests
>>> (https://en.wikipedia.org/wiki/BareMetal), would benefit from it.
>>>
>>> It would be best to introduce an option in libxl to explicitly
>>> enable/disable the vuart for DomUs. Something like vuart=1 in the VM
>>> config file.
>>
>> The vuart has not been enabled for DomU because it the UART region may clash
>> with the guest memory layout (which is static).
>>
>> I don't think this option should be available until we allow the guest to use
>> the same memory layout as the host (see e820_host parameter for x86).
>
> Actually there is no reason for the vuart to use the same address as
> the physical uart on the platform, is there?
 > In fact it doesn't even
> have to prentend to be the same uart as the one on the board, right?
> The vuart MMIO address could be completely configurable and independent
> from the one of the physical uart.

There is no reason to use the same information as the physical UART.

However, the vuart requires quite a few information (e.g base address, 
offset of different register... see vuart_info structure in 
include/xen/serial.h for more details) in order to fully work.

IHMO this is a lot of works for the user to configure. I would much 
prefer to see a PL011 emulated at a specific base address and let the 
user select whether he wants a UART to debug or not.

This would also be a start to be compliant with the VM System 
Specification (see [1]) that mandates to emulate a PL011.

>
>
>>> BTW we should keep this up to date:
>>>
>>> https://wiki.xenproject.org/wiki/Xen_ARM_TODO
>>
>> You are right, although it might be better to use the bug tracker [1] to stay
>> aligned with the rest of the hypervisor.
>>
>> Note that I have got a list of TODO/bugs I track myself but never updated the
>> wiki.
>
> I agree that we should all use the same tool, but I didn't realize there
> has been a decision on using the bug tracker for this. In fact it
> doesn't seem to be much used at the moment.

The bug tracker might be easier to update the status of feature and keep 
an history of the discussion.

>
> Maybe we should start a good old bikeshedding thread with other
> maintainers on what we should use.

I will let you kick a thread :).

Cheers,

[1] 
http://www.linaro.org/app/resources/WhitePaper/VMSystemSpecificationForARM-v2.0.pdf

-- 
Julien Grall

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