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

Gregory Haskins
2009-05-07T18:38:10Z

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Avi Kivity wrote:
> Gregory Haskins wrote:
>> I guess technically mmio can just be a simple access of the page which
>> would be problematic to trap locally without a PF. However it seems
>> that most mmio always passes through a ioread()/iowrite() call so this
>> is perhaps the hook point. If we set the stake in the ground that mmios
>> that go through some other mechanism like PFs can just hit the "slow
>> path" are an acceptable casualty, I think we can make that work.
>>
>
> That's my thinking exactly.

Cool, I will code this up and submit it. While Im at it, Ill run it
through the "nullio" ringer, too. ;) It would be cool to see the
pv-mmio hit that 2.07us number. I can't think of any reason why this
will not be the case.

>
> Note we can cheat further. kvm already has a "coalesced mmio" feature
> where side-effect-free mmios are collected in the kernel and passed to
> userspace only when some other significant event happens. We could
> pass those addresses to the guest and let it queue those writes
> itself, avoiding the hypercall completely.
>
> Though it's probably pointless: if the guest is paravirtualized enough
> to have the mmio hypercall, then it shouldn't be using e1000.

Yeah...plus at least for my vbus purposes, all my my guest->host
transitions are explicitly to cause side-effects, or I wouldn't be doing
them in the first place ;) I suspect virtio-pci is exactly the same.
I.e. the coalescing has already been done at a higher layer for
platforms running "PV" code.

Still a cool feature, tho.

-Greg

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