On Thu, May 31, 2018 at 08:43:58PM +0300, Michael S. Tsirkin wrote: > Pls work on a long term solution. Short term needs can be served by > enabling the iommu platform in qemu. So, I spent some time looking at converting virtio to dma ops overrides, and the current virtio spec, and the sad through I have to tell is that both the spec and the Linux implementation are complete and utterly fucked up. Both in the flag naming and the implementation there is an implication of DMA API == IOMMU, which is fundamentally wrong. The DMA API does a few different things: a) address translation This does include IOMMUs. But it also includes random offsets between PCI bars and system memory that we see on various platforms. Worse so some of these offsets might be based on banks, e.g. on the broadcom bmips platform. It also deals with bitmask in physical addresses related to memory encryption like AMD SEV. I'd be really curious how for example the Intel virtio based NIC is going to work on any of those plaforms. b) coherency On many architectures DMA is not cache coherent, and we need to invalidate and/or write back cache lines before doing DMA. Again, I wonder how this is every going to work with hardware based virtio implementations. Even worse I think this is actually broken at least for VIVT event for virtualized implementations. E.g. a KVM guest is going to access memory using different virtual addresses than qemu, vhost might throw in another different address space. c) bounce buffering Many DMA implementations can not address all physical memory due to addressing limitations. In such cases we copy the DMA memory into a known addressable bounc buffer and DMA from there. d) flushing write combining buffers or similar On some hardware platforms we need workarounds to e.g. read from a certain mmio address to make sure DMA can actually see memory written by the host. All of this is bypassed by virtio by default despite generally being platform issues, not particular to a given device. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
