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

[Anthony Liguori]

Avi Kivity wrote:
> 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.

I totally missed what you meant by MMIO hypercall.

In what cases do you think MMIO hypercall would result in a net benefit?

I think the difference in MMIO vs hcall will be overshadowed by the 
heavy weight transition to userspace.  The only thing I can think of 
where it may matter is for in-kernel devices like the APIC but that's a 
totally different path in Linux.

Regards,

Anthony Liguori
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