Re: Porting Xen to Jetson Nano

[Julien Grall <julien@xen.org>]

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. 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)'.

> 
> I don't think you're going to get a great deal of enthusiasm on this
> list for Xen 4.8.5, unfortunately; most people around here work off
> Xen's staging branch, and I'd be surprised to hear of anyone having
> tried a 5.7 Linux kernel with Xen 4.8.5. I can understand why you
> might start there from the existing patch series though.

Right, 4.8.5 is now out of support and we improved Xen quite a lot since 
then. As a general recommendation, I would suggest to move the series to 
the latest staging once you get it working on 4.8.5.

However, I don't see any reason why 5.7 wouldn't boot on 4.8.5. I will 
have a look at your stack trace and answer there.

Best regards,

-- 
Julien Grall