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

Gregory Haskins
2009-05-07T20:31:44Z

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Arnd Bergmann wrote:
> On Thursday 07 May 2009, 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.
>>
>> Thoughts?
>>
>
> An mmio that goes through a PF is a bug, it's certainly broken on
> a number of platforms, so performance should not be an issue there.
>

This may be my own ignorance, but I thought a VMEXIT of type "PF" was
how MMIO worked in VT/SVM. I didn't mean to imply that the guest nor
the host took a traditional PF exception in their respective IDT, if
that is what you thought I meant here. Rather, the mmio region is
unmapped in the guest MMU, access causes a VMEXIT to host-side KVM of
type PF, and the host side code then consults the guest page-table to
see if its an MMIO or not. I could very well be mistaken as I have only
a cursory understanding of what happens in KVM today with this path.

After posting my numbers today, what I *can* tell you definitively that
its significantly slower to VMEXIT via MMIO. I guess I do not really
know the reason for sure. :)
> Note that are four commonly used interface classes for PIO/MMIO:
>
> 1. readl/writel: little-endian MMIO
> 2. inl/outl: little-endian PIO
> 3. ioread32/iowrite32: converged little-endian PIO/MMIO
> 4. __raw_readl/__raw_writel: native-endian MMIO without checks
>
> You don't need to worry about the __raw_* stuff, as this should never
> be used in device drivers.
>
> As a simplification, you could mandate that all drivers that want to
> use this get converted to the ioread/iowrite class of interfaces and
> leave the others slow.
>

I guess the problem that was later pointed out is that we cannot discern
which devices might be pass-through and therefore should not be
revectored through a HC. But I am even less knowledgeable about how
pass-through works than I am about the MMIO traps, so I might be
completely off here.

In any case, thank you kindly for the suggestions.

Regards,
-Greg

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