RE: Aligning Xen to physical memory maps on embedded systems

[Levenglick Dov <Dov.Levenglick@xxxxxxxxxxxxxxxx>]

Hi Julien et al.,

> 
> (+ Stefano)
> 
> On 21/02/2021 16:30, Levenglick Dov wrote:
> > Hi,
> 
> Hi,
> 
> > 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.

I will approach their representatives. Can you comment regarding the approach 
that I
outline in the rest of the mail as though it were referring to the Xen upstream?

> > 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 working on a solution where if the "xen,domain" memory has #size-cell 
cells the
content is backward compatible. But if it contains (#address-cells + 
#size-cells), the 
address cells should be considered the physical start address.
During the mapping of the entire address space insetup_mm(), the carved out 
addresses
would be added to the  reserved memory address space. When the DomU is to be 
created,
this physical space would be mapped to it. The virtual addresses are less of an 
issue and
needn't be mapped 1x1 (although they could be).

> 
> >
> > 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.

This would be a better approach, but because Xen perform allocations from its
heap prior to allocating memory to DomU - and since it allocates from the top of
the heap - it is basically taking memory that I wanted to set aside for the 
DomU.
This is why I am thinking of reserving the memory.

> 
> >     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 embedded?
> 
> 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.

Will do. 

> 
> >
> > Thank you.
> > Dov
> 
> Cheers,
> 
> --
> Julien Grall

Thank you,
Dov


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