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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?

Yes, we have private mappings per CPU. I don't think that is as feasible on Xen, since it requires the hypervisor to support a per-CPU PD shadow for each root. But alternative implementations are possible using segmentation. The primary advantage is that you don't need to call back from the interface layer to disable preemption for per-CPU data access.

It turns out to be really easy if you add the loadsegment / savesegment macros to the VMI interface, and require the kernel to abstain from using, say, the GS segment. I think this is the path we are going down for the VMI on Xen 3 port.

I agree with your final assessment, needs more threshing out.  It does
feel a bit overkill at first blush.  I worry about these semantic
changes as an annotation instead of explicit API update.  But I guess
we still have more work on finding the right actual interface, not just
the possible ways to annotate the calls.

Yes, lets focus on finding the right interface for now - and just leave the door open a bit for the future.



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