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[Xen-devel] Re: [RFC, PATCH 1/24] i386 Vmi documentation

Chris Wright wrote:
* Zachary Amsden (zach@xxxxxxxxxx) wrote:

1) can't use stack based args, so have to allocate each data structure,
which could conceivably fail unless it's some fixed buffer.

We use a fixed buffer that is private to our VMI layer. It's a per-cpu packing struct for hypercalls. Dynamically allocating from the kernel inside the interface layer is a really great way to get into a whole lot of trouble.

Heh, indeed that's why I asked.  per-cpu buffer means ROM state knows
which vcpu is current.  How is this done in OS agnostic method w/out
trapping to hypervisor?  Some shared data that ROM and VMM know about,
and VMM updates as it schedules each vcpu?

Each VCPU gets a private data area at the same linear address. The VMM constructs private page table shadows for each VCPU, and the shadows magically contain the right mappings for that VCPU's private data area.

Other hypervisor implementations (especially those that don't make use of shadow page tables) would have to come up with something along the lines that you're suggesting.


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