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

[Julien Grall <julien.grall@linaro.org>]

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?

-- 
Julien Grall