On Mon, May 20, 2024 at 07:24:32PM +0900, Shunsuke Mie wrote: > 2024年5月16日(木) 21:59 Manivannan Sadhasivam <mani@xxxxxxxxxx>: > > > > 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. > Oops. thank you. > > > 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. > I agree, this is a problem caused by the inability to set the > capability. I'm not sure, but are there any chips that support this? Most of the recent endpoint platforms should support this feature. > Also, I wasn't aware of the IOMMU issue. I thought that if the Linux > DMA subsystem could handle IOMMU properly, there wouldn't be any > problems. Is that incorrect? The issue is, legacy virtio PCI device only has 32bits. So they cannot support VIRTIO_F_ACCESS_PLATFORM which is located at bit 33 as you explained. And if this bit is not set, then DMA APIs won't be used by the virtio stack. I think it is best to add support for modern virtio PCI device to make use of IOMMU. Legacy devices can continue to use physical address. - Mani > > Shunsuke, > Best > > - Mani > > > > -- > > மணிவண்ணன் சதாசிவம் > -- மணிவண்ணன் சதாசிவம்
