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

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Date:	Mon, 11 May 2009 20:06:14 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Hollis Blanchard <hollisb@...ibm.com>
CC:	Gregory Haskins <gregory.haskins@...il.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Gregory Haskins <ghaskins@...ell.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

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.

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