Hi virtio folks,
I'm writing to discuss finding a workaround with Virtio drivers and legacy
devices with limited memory access.
# Background
The Virtio specification defines a feature (VIRTIO_F_ACCESS_PLATFORM) to
indicate devices requiring restricted memory access or IOMMU translation. This
feature bit resides at position 33 in the 64-bit Features register on modern
interfaces. When the linux virtio driver finds the flag, the driver uses DMA
API that handles to use of appropriate memory.
# Problem
However, legacy devices only have a 32-bit register for the features bits.
Consequently, these devices cannot represent the ACCESS_PLATFORM bit. As a
result, legacy devices with restricted memory access cannot function
properly[1]. This is a legacy spec issue, but I'd like to find a workaround.
# Proposed Solutions
I know these are not ideal, but I propose the following solution.
Driver-side:
- Implement special handling similar to xen_domain.
In xen_domain, linux virtio driver enables to use the DMA API.
- Introduce a CONFIG option to adjust the DMA API behavior.
Device-side:
Due to indistinguishability from the guest's perspective, a device-side
solution might be difficult.
I'm open to any comments or suggestions you may have on this issue or
alternative approaches.
[1] virtio-net PCI endpoint function using PCIe Endpoint Framework,
https://lore.kernel.org/lkml/54ee46c3-c845-3df3-8ba0-0ee79a2acab1@xxxxxxxxxx/t/
The Linux PCIe endpoint framework is used to implement the virtio-net device on
a legacy interface. This is necessary because of the framework and hardware
limitation.
Thanks,
Shunsuke
