On Thu, May 16, 2024 at 01:38:40PM +0900, Shunsuke Mie wrote: > Hi virtio folks, > You forgot to CC the actual Virtio folks. I've CCed them now. > 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. > We can fix the endpoint framework limitation, but the problem lies with some platforms where we cannot write to vendor capability registers and still have IOMMU. - Mani -- மணிவண்ணன் சதாசிவம்
