[Xen-devel] HVM support for e820_host (Was: Bug: Limitation of <=2GB RAM in domU persists with 4.3.0)

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 09/03/2013 03:59 PM, Konrad Rzeszutek Wilk wrote:

> > > > 2) Further, I'm finding myself motivated to write that
> > > > auto-set (as opposed to hard coded) vBAR=pBAR patch discussed
> > > > briefly a week or so ago (have an init script read the BAR
> > > > info from dom0 and put it in xenstore, plus a patch to
> > > > make pBAR=vBAR reservations built dynamically rather than
> > > > statically, based on this data. Now, I'm quite fluent in C,
> > > > but my familiarity with Xen soruce code is nearly non-existant
> > > > (limited to studying an old unsupported patch every now and then
> > > > in order to make it apply to a more recent code release).
> > > > Can anyone help me out with a high level view WRT where
> > > > this would be best plumbed in (which files and the flow of
> > > > control between the affected files)?
> > > hvmloader probably and the libxl e820 code. What from a
> > > high view needs to happen is that:
> > > 1). Need to relax the check in libxl for e820_hole
> > >     to also do it for HVM guests. Said code just iterates over the
> > >     host E820 and sanitizes it a bit and makes a E820 hypercall to
> > >     set it for the guest.
[snip]

OK, I have attached a preliminary patch against 4.3.0 for the libxl 
part. It compiles. I haven't tried running it to see if it actually 
works or does something, but my packages build.
Please let me know if I've missed anything. On it's own, I don't think 
this patch will do much (apart from maybe break HVM hosts with 
e820_host=1 set).
> > > 2). Figure out whether the E820 hypercall (which sets the E820
> > >     layout for a guest) can be run on HVM guests. I think it
> > >     could not and Mukesh in his PVH patches posted a patch
> > >     to enable that - "..Move e820 fields out of pv_domain struct"
Is this already in 4.3.0 or is this an out-of-tree patch? Do you have a 
link to it handy?
> > > 2). Hvmloader should do an E820 get machine memory hypercall
> > >    to see if there is anything there. If there is - that means
> > >     the toolstack has request a "new" type of E820. Iterate
> > >     over the E820 and make it look like that.
> > >     You can look in the Linux arch/x86/xen/setup.c to see how
> > >     it does that.
> > >
> > >    The complication there is that hvmloader needs to to fit the
> > >    ACPI code (the guest type one) and such.
> > >    Presumarily you can just re-use the existing spaces that
> > >    the host has marked as E820_RESERVED or E820_ACPI..
> > Yup, I get it. Not only that, but it should also ideally (not
> > strictly necessary, but it'd be handy) map the IOMEM for devices
> > it is passed so that pBAR=vBAR (as opposed to just leaving all
> > the host e820 reserved areas well alone - which would work for
> > most things).
> Yes. That is an extra complication that could be done in subsequent
> patches. But in theory if you have the E820 mirrored from the host the
> pBAR=vBAR should be easy enough as the values from the host BARs can
> easily fit in the E820 gaps.
Agreed. Let's leave the pBAR=vBAR part for a separate patch set. I'll 
have to figure out a sensible way to query the IOMEM regions for each of 
the devices passed to the VM and make sure they are in the same hole.
> > >    Then there is the SMBIOS would need to move and the BIOS
> > >    might need to be relocated - but I think those are relocatable
> > >   in some form.
[bit above left for later reference]

> > > Well, I am more than happy to help you with this.
> > Thanks, much appreciated. :)
> Yeeey! Vict^H^H^H^volunteer :-)! <manically laughter in the background>
>
> I am also reachable on IRC (FreeNode mostly) as either darnok or konrad
> if that would be more convient to discuss this.
Thanks. I'll keep that in mind. :)

Gordan

Attachment: xen-hvm-libxl-e820_host.patch
Description: Text document

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