Re: [Xen-devel] [PATCH v2 10/10] xen/arm: add reserved-memory regions to the dom0 memory node

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 4/30/19 10:02 PM, Stefano Stabellini wrote:
> Reserved memory regions are automatically remapped to dom0. Their device
> tree nodes are also added to dom0 device tree. However, the dom0 memory
> node is not currently extended to cover the reserved memory regions
> ranges as required by the spec.  This commit fixes it.
AFAICT, this does not cover the problem mention by Amit in [1].

But I am still not happy with the approach taken for the reserved-memory 
regions in this series. As I pointed out before, they are just normal 
memory that was reserved for other purpose (CMA, framebuffer...).
Treating them as "device" from Xen POV is a clear abuse of the meaning 
and I don't believe it is a viable solution long term.
Indeed, some of the regions may have a property "reusable" allowing the 
the OS to use them until they are claimed by the device driver owning 
the region. I don't know how Linux (or any other OS) is using it today, 
but I don't see what would prevent it to use them as hypercall buffer. 
This would obviously not work because they are not actual RAM from Xen POV.
On a similar topic, because they are normal memory, I don't think 
XEN_DOMCTL_memory_mapping should be able to map reserved-regions. So the 
iomem rangeset should not contain them.
Cheers,

[1] <CABHD4K-z-x=3joJWcOb_x9m7zsjzhskDQweNBr+paLS=PFEY9Q@xxxxxxxxxxxxxx>

--
Julien Grall

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