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Re: Uses of /hypervisor memory range (was: FreeBSD/Xen/ARM issues)

Hi, all!

What do we need in order to move on on this?

It seems this can be good to have for many use-cases around

Thank you,


On 20.05.21 19:21, Stefano Stabellini wrote:
On Thu, 20 May 2021, Julien Grall wrote:
On 20/05/2021 00:25, Stefano Stabellini wrote:
On Sat, 15 May 2021, Julien Grall wrote:
My feeling is one of two things should happen with the /hypervisor
address range:

1>  OSes could be encouraged to use it for all foreign mappings.  The
range should be dynamic in some fashion.  There could be a handy way to
allow configuring the amount of address space thus reserved.
In the context of XSA-300 and virtio on Xen on Arm, we discussed about
providing a revion for foreign mapping. The main trouble here is figuring
of the size, if you mess it up then you may break all the PV drivers.

If the problem is finding space, then I would like to suggest a different
approach (I think I may have discussed it with Andrew). Xen is in
the P2M for the guest and therefore now where are the unallocated space.
about introducing a new hypercall to ask for "unallocated space"?

This would not work for older hypervisor but you could use the RAM instead
Linux does). This is has drawbacks (e.g. shattering superpage, reducing
amount of memory usuable...), but for 1> you would also need a hack for
I am starting to wonder if it would make sense to add a new device tree
binding to describe larger free region for foreign mapping rather then a
hypercall. It could be several GBs or even TBs in size. I like the idea
of having it device tree because after all this is static information.
I can see that a hypercall would also work and I am open to it but if
possible I think it would be better not to extend the hypercall
interface when there is a good alternative.
There are two issues with the Device-Tree approach:
   1) This doesn't work on ACPI
   2) It is not clear how to define the size of the region. An admin doesn't
have the right information in hand to know how many mappings will be done (a
backend may map multiple time the same page).

The advantage of the hypercall solution is the size is "virtually" unlimited
and not based on a specific firmware.
Fair enough



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