On Fri, Oct 02, 2020 at 04:26:10PM +0200, Arnd Bergmann wrote:
At the moment the bounce buffer is allocated from a global pool in the low physical pages. However a recent proposal by Chromium would add support for per-device swiotlb pools:
https://lore.kernel.org/linux-iommu/20200728050140.996974-1-tientzu@chromium...
And quoting Tomasz from the discussion on patch 4:
For this, I'd like to propose a "restricted-dma-region" (feel free to suggest a better name) binding, which is explicitly specified to be the only DMA-able memory for this device and make Linux use the given pool for coherent DMA allocations and bouncing non-coherent DMA.Right, I think this can work, but there are very substantial downsides to it:
- it is a fairly substantial departure from the virtio specification, which defines that transfers can be made to any part of the guest physical address space
Coming back to this point, that was originally true but prevented implementing hardware virtio devices or putting a vIOMMU in front of the device. The spec now defines feature bit VIRTIO_F_ACCESS_PLATFORM (previously called VIRTIO_F_IOMMU_PLATFORM):
A device SHOULD offer VIRTIO_F_ACCESS_PLATFORM if its access to memory is through bus addresses distinct from and translated by the platform to physical addresses used by the driver, and/or if it can only access certain memory addresses with said access specified and/or granted by the platform. A device MAY fail to operate further if VIRTIO_F_ACCESS_PLATFORM is not accepted.
With this the driver has to follow the DMA layout given by the platform, in our case given by the device tree.
Another point about solution #1: since the backend (secondary VM) accesses virtqueue directly (we probably want to avoid a scatter-gather translation step in the backend) the pointers written by the frontent in the virtqueue have to be guest-physical addresses of the backend. So the static memory region needs to have identical guest-physical address in primary and secondary VM. In practice I doubt this would be a problem.
Thanks, Jean