Re: [Xen-devel] Proposed new "memory capacity claim" hypercall/feature

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Tim Deegan [mailto:tim@xxxxxxx]
> Subject: Re: Proposed new "memory capacity claim" hypercall/feature

Oops, missed an important part of your response... I'm glad
I went back and reread it...
 
> The claim hypercall _might_ fix (c) (if it could handle allocations that
> need address-width limits or contiguous pages).

I'm still looking into this part.

It's my understanding (from Jan) that, post-dom0-launch, there are
no known memory allocation paths that _require_ order>0 allocations.
All of them attempt a larger allocation and gracefully fallback
to (eventually) order==0 allocations.  I've hacked some code
in to the allocator to confirm this, though I'm not sure how
to test the hypothesis exhaustively.

For address-width limits, I suspect we are talking mostly or
entirely about DMA in 32-bit PV domains?  And/or PCI-passthrough?
I'll look into it further, but if those are the principal cases,
I'd have no problem documenting that the claim hypercall doesn't
handle them and attempts to build such a domain might still
fail slowly.  At least unless/until someone decided to add
any necessary special corner cases to the claim hypercall.


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