Re: [Xen-devel] [PATCH v3 12/13] xen/arm: Add the property "protected-devices" in the hypervisor node

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-04-04 at 14:10 +0100, Julien Grall wrote:
> On 04/04/2014 01:45 PM, Ian Campbell wrote:
> > On Fri, 2014-04-04 at 12:23 +0100, Julien Grall wrote:
> >> On 04/04/2014 12:13 PM, Ian Campbell wrote:
> >>> We could enable both of those things only at the point where vm_assist
> >>> was called.
> >>
> >> [..]
> >>
> >>> I think we can reasonably require that vm_assist be called super early,
> >>> i.e. before any DMA operations occur, in Linux terms I think
> >>> early_initcall(xen_guest_init) is likely early enough, or we could move
> >>> xen_guestr_init even sooner.
> >>
> >> IHMO, this solution is far too complicate and we will have to fork from
> >> the current IOMMU framework.
> > 
> > Eh? On the Linux side? You just call this hypercall super early so that
> > you are sure no drivers have initialised yet and you are done, there is
> > no need to fork anything AFAICT. And in any case if any generic
> > framework needs extending to better work with Xen then we should do
> > that, not fork it or make a worse design to sidestep the need.
> > 
> > Obviously there will be complexity on the Xen side, but I think far too
> > complicated will be an overstatement.
> 
> I was talking about Xen side. The current IOMMU platform would need some
> rework to support it.
> 
> For me this solution is far too complicate compare to the other
> solutions (e.g ELF note).
> 
> > 
> >>> We also have the option of a build time feature flag in the image itself
> >>> like on x86. It is likely that going forward we will have other cases
> >>> where we wished we had such a thing so getting it in place now would be
> >>> useful.
> >>
> >> I definitely prefer this solution which seems more cleaner than the others.
> >>
> >> Is it possible to embedded notes in a zImage? How x86 handle other
> >> format than ELF?
> > 
> > The x86 bzImage format embeds an ELF file, which something which was
> > done long ago to improve things for Xen PV x86 use which wants to boot
> > an ELF.
> > 
> > ARM zImage doesn't currently have anything like that so you would be
> > looking at an image format extension.
> 
> Ok. So it will be easy to add support for ELF image. However for zImage,
> do you think Linux ARM maintainers will accept a change in the format?

I've no idea. I'm 100% positive that they will require it to be done in
a backwards compatible way. I don't know how amenable the zImage format
is to such extensions.

On x86 IIRC the kexec people were interested in being able to get an ELF
out of a bzImage, you might find that the same is true on ARM, which
might help with motivating the change.

Ian.


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