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Subject: Re: Constraining where a guest may allocate virtio accessible resources

On Wed, Jun 17, 2020 at 06:31:15PM +0100, Alex BennÃe 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.


> 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?

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