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

[Avi Kivity]

Gregory Haskins wrote:
> 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)
> >
> >     
> Yeah, I agree with all this.  I am still wrestling with how to deal with
> the device-assignment problem w.r.t. shunting io requests into a
> hypercall vs letting them PF.  Are you saying we could simply ignore
> this case by disabling "MMIOoHC" when assignment is enabled?  That would
> certainly make the problem much easier to solve.
>   

No, we need to deal with hotplug.  Something like IO_COND that Chris 
mentioned, but how to avoid turning this into a doctoral thesis.

(On the other hand, device assignment requires the iommu, and I think 
you have to specify that up front?)

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

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