RE: Aligning Xen to physical memory maps on embedded systems

[Levenglick Dov <Dov.Levenglick@xxxxxxxxxxxxxxxx>]

Thank you.
A few final comments below + one last question regarding the Xilinx forks:
Xen 4.13 is first available on the 2020.1 branch. Is it required that the 
2020.1 branch of linux-xlnx be used as well, or can I keep the 2019.1 branch 
that I am currently using?


> -----Original Message-----
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> Sent: Tuesday, March 2, 2021 3:42 AM
> To: Levenglick Dov <Dov.Levenglick@xxxxxxxxxxxxxxxx>
> Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Xen-
> users@xxxxxxxxxxxxxxxxxxxx; Wei Chen <Wei.Chen@xxxxxxx>; Penny Zheng
> <Penny.Zheng@xxxxxxx>; Luca Fancellu <Luca.Fancellu@xxxxxxx>
> Subject: RE: Aligning Xen to physical memory maps on embedded systems
> 
> On Mon, 1 Mar 2021, Levenglick Dov wrote:
> > > (+ Penny, Wei and Luca)
> > >
> > > > On 23 Feb 2021, at 01:52, Stefano Stabellini <sstabellini@xxxxxxxxxx>
> wrote:
> > > >
> > > > On Mon, 22 Feb 2021, Levenglick Dov wrote:
> > > >>>> 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).
> > > >
> > > > As of today neither upstream Xen nor the Xilinx Xen tree come with
> > > > the feature of allowing the specification of an address range for
> > > > dom0less guests.
> > > >
> > > > The only thing that Xilinx Xen allows, which is not upstream yet,
> > > > is the ability of creating dom0less guests 1:1 mapped using the "direct-
> map"
> > > > property. But the memory allocation is still done by Xen (you
> > > > can't select the addresses).
> > > >
> > > > Some time ago I worked on a hacky prototype to allow the
> > > > specification of address ranges, see:
> > > >
> > > > http://xenbits.xenproject.org/git-http/people/sstabellini/xen-unst
> > > > able .git direct-map-2 from
> > > > 7372466b21c3b6c96bb7a52754e432bac883a1e3
> > > onward.
> > > >
> > > > In particular, have a look at "xen/arm: introduce 1:1 mapping for
> > > > domUs". The work is not complete: it might not work depending on
> > > > the memory ranges you select for your domUs. In particular, you
> > > > can't select top-of-RAM addresses for your domUs. However, it
> > > > might help you getting started.
> > > >
> > > >
> > > >>>> 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.
> > > >
> > > > Yeah, this is the main problem that my prototype above couldn't solve.
> >
> > Stephano: Is the approach that I previously described a feasible one?
> >   1. Mark the addresses that I want to set aside as reserved
> >   2. When reaching the proper DomU, map them and then use the mapping
> > This approach would solve the heap issue
> 
> My first suggestion would be actually to let the hypervisor pick the address
> ranges. If you don't change setup, you'll see that they are actually stable
> across reboot. WARNING: Xen doesn't promise that they are stable;
> however, in practice, they are stable unless you change device tree or
> configuration or software versions.
> 
> That said, yes, I think your approach might work with some limitations (e.g.
> Xen reclaiming memory on domU destruction but you probably don't care
> about that). It could be a decent stopgap until we get a better solution.

Is DomU destruction an option on true Dom0-less? Who would be doing the 
destruction?

> From a Xen upstream point of view, it makes sense to follow the approach
> used by Penny, Wei, and Betrand that seems to be the one that is more
> flexible and integrate better with the existing codebase.

I will wait for their response regarding commits and backporting.

> > > Wei and Penny are working on direct map and static allocation to fit
> > > embedded use cases an might have more answer there.
> >
> > Bertrand, Wei and Penny: Is there a "sneak preview"? I'd be happy to
> > start backporting to Xen 4.11
> 
> As mentioned, there is a 4.13-based Xilinx Xen tree available too.
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