Re: [Xen-devel] How to boot domU and dom0 from a device tree

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 12 Jun 2019, Julien Grall wrote:
> (Moving from xen-users to xen-devel).
> 
> On 11/06/2019 23:18, Stefano Stabellini wrote:
> > I managed to reproduced the issue, and I know how to get past it.  Try
> > using the raw kernel Image (arch/arm64/boot/Image) instead of Image.gz
> > for dom0 and domU. That fixed it for me.
> > 
> > Julien, I didn't manage to figure out what the issue is exactly, but it
> > looks like Image.gz loading is broken at the moment.
> 
> Do you mean Image.gz is broken from DomU? Because per the log provided by
> Denis, this is working perfectly for Dom0 as we don't create domain in
> parallel.
> 
> By reading the code I can already spot the reason of the first issue reported
> by Denis. For reminder, this is when Dom0 and DomU are using the same module
> address for the gzip Image.
> 
> This is because when probing the kernel for Dom0, the module will get
> uncompressed and the module start/end will be updated to point to the
> uncompress version. Because of that, the probe for DomU kernel will not be
> able to find the module (the start addressed changed).
> 
> In this case, I think we only want to uncompress the module one time to avoid
> wasting memory. The solution I have in mind requires some rework in Xen, I
> would actually start by probing the information for all the domains, then
> uncompress the kernels modules, and then finish to build the domain.
> 
> For the out of memory problem discussed in this e-mail, I think the problem is
> not because of lack of memory in DomU. The problem is related to the
> inflate/gunzip the code. The code is using an heap (see perform_gunzip) where
> it allocates memory from.
> 
> I am assuming the kernels for Dom0 and DomU are exactly the same but they are
> coming from different address. Am I correct? If so, I am a bit unsure this
> worked the first time and not the second time. This probably want some
> debugging to understand the problem. Denis, Stefano, can one of you look at
> it?

I couldn't find exactly the root cause yet, but I can reproduce the
issue even with Dom0 only (no domUs, no dom0less):

(XEN) *** LOADING DOMAIN 0 ***
(XEN) DEBUG kernel_probe 445
(XEN) Loading d0 kernel from boot module @ 0000000047000000
(XEN) Loading ramdisk from boot module @ 0000000042000000
(XEN) DEBUG kernel_decompress 268
(XEN) DEBUG kernel_decompress 272
(XEN) DEBUG kernel_decompress 279
(XEN) DEBUG kernel_decompress 284
(XEN) DEBUG kernel_decompress 291 kernel_order_out=52 output_size=0
(XEN) 
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Could not set up DOM0 guest OS
(XEN) ****************************************
(XEN) 
(XEN) Reboot in five seconds...


The issue seems to be that output_size, returned by output_length(input,
size) is 0. Then, kernel_order_out is set to 52 which is too large. As a
consequence kernel_decompress returns with -ENOMEM.

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