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

[Daniel Arai <arai@xxxxxxxxxx>]

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.
Dan.

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