On Mon, Jun 11, 2018 at 01:29:18PM +1000, Benjamin Herrenschmidt wrote: > On Sun, 2018-06-10 at 19:39 -0700, Ram Pai wrote: > > > > However if the administrator > > ignores/forgets/deliberatey-decides/is-constrained to NOT enable the > > flag, virtio will not be able to pass control to the DMA ops associated > > with the virtio devices. Which means, we have no opportunity to share > > the I/O buffers with the hypervisor/qemu. > > > > How do you suggest, we handle this case? > > At the risk of repeating myself, let's just do the first pass which is > to switch virtio over to always using the DMA API in the actual data > flow code, with a hook at initialization time that replaces the DMA ops > with some home cooked "direct" ops in the case where the IOMMU flag > isn't set. I'm not sure I understand all of the details, will have to see the patch, but superficially it sounds good to me. > This will be equivalent to what we have today but avoids having 2 > separate code path all over the driver. > > Then a second stage, I think, is to replace this "hook" so that the > architecture gets a say in the matter. > > Basically, something like: > > arch_virtio_update_dma_ops(pci_dev, qemu_direct_mode). > > IE, virtio would tell the arch whether the "other side" is in fact QEMU > in a mode that bypasses the IOMMU and is cache coherent with the guest. > This is our legacy "qemu special" mode. If the performance is > sufficient we may want to deprecate it over time and have qemu enable > the iommu by default but we still need it. > > A weak implementation of the above will be provied that just puts in > the direct ops when qemu_direct_mode is set, and thus provides today's > behaviour on any arch that doesn't override it. If the flag is not set, > the ops are left to whatever the arch PCI layer already set. > > This will provide the opportunity for architectures that want to do > something special, such as in our case, when we want to force even the > "qemu_direct_mode" to go via bounce buffers, to put our own ops in > there, while retaining the legacy behaviour otherwise. > > It also means that the "gunk" is entirely localized in that one > function, the rest of virtio just uses the DMA API normally. > > Christoph, are you actually hacking "stage 1" above already or should > we produce patches ? > > Cheers, > Ben. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
