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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
