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Re: [Xen-devel] [v7][RFC][PATCH 06/13] hvmloader/ram: check if guest memory is out of reserved device memory maps



>>> On 20.11.14 at 08:45, <kevin.tian@xxxxxxxxx> wrote:
> Yang and I did some discussion here. We understand your point to
> avoid introducing new interface if we can leverage existing code.
> However it's not a trivial effort to move device assignment before 
> populating p2m, and there is no other benefit of doing so except
> for this purpose. So we'd not suggest this way.

"It's not a trivial effort" is pretty vague: What specifically is it that
makes this difficult? I wouldn't expect there to be any strong
dependencies on the ordering of these two operations...

> Current option sounds a reasonable one, i.e. passing a list of BDFs
> assigned to this VM before populating p2m, and then having 
> hypervisor to filter out reserved regions associated with those 
> BDFs. This way libxc teaches Xen to create reserved regions once,
> and then later the filtered info is returned upon query.

Reasonable, but partly redundant. The positive aspect being that
it permits this list and the list of actually being assigned devices to
be different, i.e. allowing holes to be set up for devices that only
_may_ get assigned at some point.

> The limitation of wasted memory due to confliction can be
> mitigated, and we considered further enhancement can be made
> later in libxc that when populating p2m, the reserved regions
> can be skipped explicitly at initial p2m creation phase and then 
> there would be no waste at all. But this optimization takes some
> time and can be built incrementally on current patch and interface, 
> post 4.5 release. For now let's focus on the very correctness first.

I agree, as long as the optimization part doesn't get dropped after
the correctness part went in.

Jan


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