On Wed, Jun 17, 2020 at 06:31:15PM +0100, Alex Bennée wrote: [...] > Option 2 - Additional Platform Data > =================================== > > This would be extending using something like device tree or ACPI tables > which could define regions of memory that would inform the low level > memory allocation routines where they could allocate from. There is > already of the concept of "dma-ranges" in device tree which can be a > per-device property which defines the region of space that is DMA > coherent for a device.
They are regions that are accessible to a device for DMA, coherency is described through other methods. Thinking more about this, dma-ranges (and ACPI _DMA) don't exactly describe what you need. They describe addressing limitation from a bridge's perspective, for example from the PCI root complex. So there are at least two issues: 1. They apply to the whole downstream bus, so you can't define per-device DMA windows. Although with PCIe I suppose you could put one on each downstream port. 2. More importantly, they only describe addressing limitations locally. When the device directly accesses memory, it emits guest-physical addresses (GPA) so you can use DMA ranges to describe which memory it can access. However, if there is an IOMMU in between, the device emits I/O virtual addresses (IOVA), which are translated by the IOMMU into GPA. In this case the DMA ranges apply to the IOVA, and there doesn't exist a way to describe limitations on the GPA. There are other mechanisms describing addressing limitations such as Intel's RMRR, but those also apply to IOVAs as far as I know. Thanks, Jean > > There is the question of how you tie regions declared here with the > eventual instantiating of the VirtIO devices? > > For a fully distributed set of backends (one backend per device per > worker VM) you would need several different regions. Would each region > be tied to each device or just a set of areas the guest would allocate > from in sequence? --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org