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

Avi Kivity
2009-05-11T17:07:58Z

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)

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