On Tue, Apr 20, 2021 at 08:01:06PM -0400, Kyle Harding wrote: > On Tue, Apr 20, 2021 at 3:51 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > We'd like to essentially run a virtualized vendor kernel to manage a > > > hardware device such as an SDIO wireless card passed through from the > host, > > > running a mainline kernel. Network traffic would be routed from host to > > > guest over a virtio interface. > > > > > > Is it possible to use Greybus in this way, or am I misunderstanding the > use > > > case of this subsystem? > > > > You could abuse greybus to do something like this, but that feels like a > > ton of extra work when there are already solutions for this type of > > thing today in Linux. Why not just use some of the existing hardware > > pass-through solutions that are out there that allow virtual kernels to > > have access to hardware directly? Why try to create yet-another way to > > do this? > > > > But hey, if you want to take the time to write such kernel code, all the > > best! :) > > > > greg k-h > > We've explored options such as VFIO for this use case. Although VFIO works > great on PCs with hardware peripherals on an I/O bus such as PCI behind an > IOMMU, we're looking to passthrough a platform device (SDIO wireless > specifically, but potentially others as well) on an embedded system, such > as a Raspberry Pi 4. If you are using hardware as cheap and broken as a rpi, sorry, you are on your own here. The processor in that thing is not set up for any type of virtualization, so to try to make it work that way is a lot of work for no gain. > We've investigated vfio-platform intended for this, but based on a paper > [1] describing the architecture and use cases, it seems to require specific > hardware that implements DMA translation functionality, such as an IOMMU or > SMMU, along with the appropriate driver. As we understand, a lot of the > platforms we support don't have an IOMMU. Try it and see, who knows? You know your use case better than anyone else, and only by trying stuff will you determine what actually needs to be done. > The kernel docs [2] describe the programming interface of devices as > generally composed of I/O access, interrupts, and DMA. > > Devices are the main target of any I/O driver. Devices typically > create a programming interface made up of I/O access, interrupts, > and DMA. Without going into the details of each of these, DMA is > by far the most critical aspect for maintaining a secure environment > as allowing a device read-write access to system memory imposes the > greatest risk to the overall system integrity. > > This seems to explain the need for an IOMMU to passthrough a device to a > virtualized guest, as any DMA done by a device would need guest > physical-addresses translated to host-physical addresses. > However, we don't know for certain if the driver for our wireless adapter > (mwifiex) uses DMA. Grepping the source shows references to DMA, but the > fact that SDIO devices can be connected over a USB bridge seems to indicate > it either doesn't, or we're not completely understanding. Odds are the driver itself does not require DMA, that comes from the layers below the driver that talks to the specific hardware on the platform. And if your device is connected over a USB bridge, that shows that the driver doesn't matter here, it's up to the bus controller to do that type of work for you. > We really don't know how feasible this idea is given our hardware > constraints. Greybus may very likely be solving an entirely different > issue, but it seemed to have enough overlap to be worth investigating. > We're looking to understand if this is possible given our hardware > constraints, or if we're barking up the wrong tree. No idea, try it and see, good luck! You have a long road ahead of you, I really would recommend using sane hardware... greg k-h _______________________________________________ greybus-dev mailing list greybus-dev@xxxxxxxxxxxxxxxx https://lists.linaro.org/mailman/listinfo/greybus-dev
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