Messages in this thread | | | Date | Wed, 13 Jun 2018 16:49:39 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices |
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On Thu, Jun 07, 2018 at 11:36:55PM -0700, Christoph Hellwig wrote: > > This seems to be what was being asked for in this thread, > > with comments claiming IOMMU flag adds too much overhead. > > Right now it means implementing a virtual iommu, which I agree is > way too much overhead.
Well not really. The flag in question will have a desired effect without a virtual iommu.
> > SEV guys report that they just set the iommu flag and then it all works. > > I guess if there's translation we can think of this as a kind of iommu. > > Maybe we should rename PLATFORM_IOMMU to PLARTFORM_TRANSLATION? > > VIRTIO_F_BEHAVES_LIKE_A_REAL_PCI_DEVICE_DONT_TRY_TO_OUTSMART_ME > > as said it's not just translations, it is cache coherence as well.
Well it's only for DMA. So maybe PLATFORM_DMA.
I suspect people will then come and complain that they do *not* want cache coherence tricks because virtio is running on a CPU, but we'll see.
> > And apparently some people complain that just setting that flag makes > > qemu check translation on each access with an unacceptable performance > > overhead. Forcing same behaviour for everyone on general principles > > even without the flag is unlikely to make them happy. > > That sounds like a qemu implementation bug. If qemu knowns that > guest physiscall == guest dma space there is no need to check.
Possibly. Or it's possible it's all just theoretical, no one posted any numbers.
-- MST
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