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Re: Aligning Xen to physical memory maps on embedded systems

(+ Stefano)

On 21/02/2021 16:30, Levenglick Dov wrote:


I am booting True Dom0-less on Xilinx ZynqMP UltraScale+ using Xen 4.11,
taken from https://github.com/Xilinx/xen.

This tree is not an official Xen Project tree. I can provide feedback based on how Xen upstream works, but I don't know for sure if this will apply to the Xilinx tree.

For any support, I would recommend to contect Xilinx directly.

The system has 2GB of RAM (0x00000000 - 0x80000000) of which Xen and the DomU
have an allocation of 1.25GB, per this memory map:
1. DomU1: 0x60000000 - 0x80000000
2. DomU2: 0x40000000 - 0x60000000
3. Xen: 0x30000000 - 0x40000000

How did you tell Xen which regions is assigned to which guests? Are your domain mapped 1:1 (i.e guest physical address == host physical address)?

I am able to support True Dom0-less by means of the patch/hack demonstrated
By Stefano Stabellini at https://youtu.be/UfiP9eAV0WA?t=1746.

I was able to forcefully put the Xen binary at the address range immediately
below 0x40000000 by means of modifying get_xen_paddr() - in itself an ugly hack.

My questions are:
1. Since Xen performs runtime allocations from its heap, it is allocating
    downwards from 0x80000000 - thereby "stealing" memory from DomU1.

In theory, any memory reserved for domains should have been carved out from the heap allocator. This would be sufficient to prevent Xen allocating memory from the ranges you described above.

Therefore, to me this looks like a bug in the tree you are using.

    Can I force the runtime allocations to be from a specific address range?
2. Has the issue of physical memory map address maps been addressed by Xen for 

Xen 4.12+ will not relocate itself to the top of the memory anymore. Instead, it will stay where it was first loaded in memory.

I would recommend to ask Xilinx if they can provide you with a more recent tree.

Thank you.
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Julien Grall



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