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Re: [Xen-devel] [PATCH v3 3/3] xen: introduce XENMEM_get_dma_buf and XENMEM_put_dma_buf

On Thu, 8 Aug 2013, Ian Campbell wrote:
> >  The new pages are going to be "pinned": it's guaranteed
> > + * that their p2m mapping won't be changed until explicitly "unpinned".
> We have a privilege level check here through the use of
> multipage_allocation_permitted, which permits up to 2MB mappings. But
> unlike the original exchange these can be cumulative. Do we need a per
> domain limit on the number of pinned p2m pages? My gut feeling is yes,
> otherwise a guest can pin 1 page in every 2MB range, which isn't so bad
> now but will cause fragementation issues when we switch to 2MB p2m
> mappings.

Wouldn't it cause fragementation issues only to itself?
As far as I can tell the only problem that pinned p2m pages can cause to
the hypervisor is preventing Xen from swapping or sharing domain pages.

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