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

[Anthony Liguori]

Avi Kivity wrote:
> Hollis Blanchard wrote:
> > I haven't been following this conversation at all. With that in mind...
> >
> > AFAICS, a hypercall is clearly the higher-performing option, since you
> > don't need the additional memory load (which could even cause a page
> > fault in some circumstances) and instruction decode. That said, I'm
> > willing to agree that this overhead is probably negligible compared to
> > the IOp itself... Ahmdal's Law again.
> >   
>
> 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.

Regards,

Anthony Liguori
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