Messages in this thread | | | Date | Mon, 11 May 2009 20:53:32 +0300 | From | Avi Kivity <> | Subject | Re: [RFC PATCH 0/3] generic hypercall support |
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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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