Re: [RFC PATCH 0/3] generic hypercall support

[Avi Kivity]

Anthony Liguori wrote: 
> > It's a question of cost vs. benefit.  It's clear the benefit is low 
> > (but that doesn't mean it's not worth having).  The cost initially 
> > appeared to be very low, until the nested virtualization wrench was 
> > thrown into the works.  Not that nested virtualization is a reality 
> > -- even on svm where it is implemented it is not yet production 
> > quality and is disabled by default.
> >
> > Now nested virtualization is beginning to look interesting, with 
> > Windows 7's XP mode requiring virtualization extensions.  Desktop 
> > virtualization is also something likely to use device assignment 
> > (though you probably won't assign a virtio device to the XP instance 
> > inside Windows 7).
> >
> > Maybe we should revisit the mmio hypercall idea again, it might be 
> > workable if we find a way to let the guest know if it should use the 
> > hypercall or not for a given memory range.
> >
> > mmio hypercall is nice because
> > - it falls back nicely to pure mmio
> > - it optimizes an existing slow path, not just new device models
> > - it has preexisting semantics, so we have less ABI to screw up
> > - for nested virtualization + device assignment, we can drop it and 
> > get a nice speed win (or rather, less speed loss)
>
> If it's a PCI device, then we can also have an interrupt which we 
> currently lack with vmcall-based hypercalls.  This would give us 
> guestcalls, upcalls, or whatever we've previously decided to call them.

Sorry, I totally failed to understand this.  Please explain.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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