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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