Re: Porting Xen to Jetson Nano

[Julien Grall <julien@xen.org>]

On 23/07/2020 18:04, Julien Grall wrote:
> Hi,
> 
> On 23/07/2020 05:26, Christopher Clark wrote:
>> On Wed, Jul 22, 2020 at 10:59 AM Srinivas Bangalore <srini@yujala.com> 
>> wrote:
>>> Dear Xen experts,
>>>
>>> Would greatly appreciate some hints on how to move forward with this 
>>> one…
>>
>> Hi Srini,
>>
>> I don't have any strong recommendations for you, but I do want to say
>> that I'm very happy to see you taking this project on and I am hoping
>> for your success. I have a newly-arrived Jetson Nano sitting on my
>> desk here, purchased with the intention of getting Xen up and running
>> on it, that I just haven't got to work on yet. I'm also familiar with
>> Chris Patterson, Kyle Temkin and Ian Campbell's previous Tegra Jetson
>> patches and it would be great to see some further progress made from
>> those.
> 
> I agree that it would be good to have the support in upstream!
> 
>>
>> In my recent experience with the Raspberry Pi 4, one basic observation
>> with ARM kernel bringup is that if your device tree isn't good, your
>> dom0 kernel can be missing the configuration it needs to use the
>> serial port correctly and you don't get any diagnostics from it after
>> Xen attempts to launch it, so I would just patch the right serial port
>> config directly into your Linux kernel (eg. hardcode specific things
>> onto the kernel command line) so you're not messing about with that
>> any more.
>>
>> The other thing I would recommend is patching in some printks into the
>> earliest part of the Xen parts of the Dom0 Linux kernel start code.
>> Others who are more familar with Xen on ARM may have some better
>> recommendations, but linux/arch/arm/xen/enlighten.c has a function
>> xen_guest_init that looks like a good place to stuff some extra
>> printks for some early proof-of-entry from your kernel, and that way
>> you'll have some indication whether execution has actually commenced
>> in there.
> 
> Linux provides earlyprintk facilities that can be used in Xen. To enable 
> it, you need to have your kernel built with CONFIG_EARLY_PRINTK=y and 
> CONFIG_XEN=y. This can then be enabled by passing earlyprintk=xenboot on 
> your command line.
> 
> Note that Linux needs to detect you are using Xen before using 
> earlyprintk.

Hmmm... I forgot to earlyprintk is x86 only. On Arm, you want to use 
earlycon=xenboot. This should be available as long as you build Linux 
with CONFIG_XEN=y. No need to detect Xen. However...

> If you need earlier, then what I usually do is hacking 
> xen_raw_console_write() (in drivers/tty/hvc/hvc_xen.c) and replace 'if 
> (xen_domain())' with 'if (1)'.

... this point is still valid if you want to use earlycon before it is 
actually initialized.

Apologies for the confusion.

Cheers,

-- 
Julien Grall