RE: Aligning Xen to physical memory maps on embedded systems

[Levenglick Dov <Dov.Levenglick@xxxxxxxxxxxxxxxx>]

Thank you all

Dov


> -----Original Message-----
> From: Wei Chen <Wei.Chen@xxxxxxx>
> Sent: Wednesday, March 3, 2021 8:43 AM
> To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>; Levenglick Dov
> <Dov.Levenglick@xxxxxxxxxxxxxxxx>
> Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Xen-
> users@xxxxxxxxxxxxxxxxxxxx; Penny Zheng <Penny.Zheng@xxxxxxx>; Luca
> Fancellu <Luca.Fancellu@xxxxxxx>
> Subject: RE: Aligning Xen to physical memory maps on embedded systems
> 
> Hi,
> 
> Sorry to reply late. This e-mail had been filtered by my e-mail client.
> We have been working on direct mapping and static allocation for a while.
> And Penny had sent an initial version of direct mapping design document to
> mailing list in last December.
> 
> Now, we are working on a new version design document, the new version
> design will address the feedbacks we have got from the initial version and
> will also include the design of static allocation. This document is nearing
> completion and we will be submitting it to the community for discussion in
> the next week or two. Once we have some conclusions, we will soon be
> submitting our code to the community to collect the RFC.
> 
> Besides, I have some comments below:
> 
> > -----Original Message-----
> > From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> > Sent: 2021年3月3日 3:36
> > To: Levenglick Dov <Dov.Levenglick@xxxxxxxxxxxxxxxx>
> > Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>; 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 Tue, 2 Mar 2021, Levenglick Dov wrote:
> > > 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?
> >
> > Xilinx recommends to always use the same version everywhere, so 2020.1
> > for Xen, Linux, firwmare, etc.
> >
> > That said, it should be no problem to use Xen 2020.1 with everything
> > else from 2019.1. Given that you are using dom0less, you just need to
> > rebuild the Xen hypervisor alone, you don't even need to update the
> > dom0 rootfs.
> >
> >
> > > > -----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/xe
> > > > > > > n-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.
> > >
> 
> In our new design, the user defined memory ranges for DomU and memory
> reclaiming on DomU destruction have been considered already. These are
> two features that we really want the community to discuss and get feedback
> on.
> 
> > > Is DomU destruction an option on true Dom0-less? Who would be doing
> > > the
> > destruction?
> >
> > Destruction, yes. You should be able to use "xl destroy" in Dom0
> > already today to destroy a dom0less domU. Pass a domid instead of
> > domain name (they don't have a domain name). Of course you need the xl
> > tools in the Xen rootfs for that, so if you are going to update Xen,
> > then you also need to update the Xen tools, hence the Dom0 rootfs. The
> > Xen tools and Xen actually need to be of the same version.
> >
> > If you intend to create again a dom0less domain after destroying it
> > (reboot), then you need to have a config file in dom0 with the same
> > configuration so that you can call xl create.
> 
> Cheers,
> Wei Chen
> 

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