On Wed, 2014-02-12 at 23:51 +0100, Benoît Canet wrote: > The Wednesday 12 Feb 2014 à 12:34:25 (-0700), Alex Williamson wrote : > > On Wed, 2014-02-12 at 19:10 +0100, Benoît Canet wrote: > > > Hi Alex, > > > > > > After the IRC conversation we had a few days ago I understood that guest > > > IOMMU > > > was not implemented. > > > > > > I have a real use case for it: > > > > > > Cisco usnic allow to write MPI applications while driving the network > > > card in > > > userspace in order to optimize the latency. It's made for compute > > > clusters. > > > > > > The typical cloud provider don't provide bare metal access but only vms > > > on top > > > of Cisco's hardware hence VFIO is using the IOMMU to passthrough the NIC > > > to the > > > guest and no IOMMU is present in the guest. > > > > > > questions: Would writing a performing guest IOMMU implementation be > > > possible ? > > > How complex this project looks for someone knowing IOMMUs > > > issues ? > > > > > > The ideal implementation would forward the IOMMU work to the host > > > hardware for > > > speed. > > > > > > I can devote time writing the feature if it's doable. > > > > Hi Benoît, > > > > I imagine it's doable, but it's certainly not trivial, beyond that I > > haven't put much thought into it. > > > > VFIO running in a guest would need an IOMMU that implements both the > > IOMMU API and IOMMU groups. Whether that comes from an emulated > > physical IOMMU (like VT-d) or from a new paravirt IOMMU would be for you > > to decide. VT-d would imply using a PCIe chipset like Q35 and trying to > > bandage on VT-d or updating Q35 to something that natively supports > > VT-d. Getting a sufficiently similar PCIe hierarchy between host an > > guest would also be required. > > This Cisco thing usnic (driver/infiniband/hw/usnic) does not seems to use VFIO > at all and seems to be hardcoded to make use of an intel IOMMU. > > I don't know if it's a good thing or not.
Sorry, I got a little off track assuming usnic was a VFIO userspace driver. Peeking quickly at it, it looks like it also uses the IOMMU API, so unless I missed the VT-d specific parts, a pv IOMMU in the guest might allow some simplification if you don't care about non-Linux support. > > The current model of putting all guest devices in a single IOMMU domain > > on the host is likely not what you would want and might imply a new VFIO > > IOMMU backend that is better tuned for separate domains, sparse > > mappings, and low-latency. VFIO has a modular IOMMU design, so this > > isn't architecturally a problem. The VFIO user (QEMU) is able to select > > which backend to use and the code is written with supporting multiple > > backends in mind. > > > > A complication you'll have is that the granularity of IOMMU operations > > through VFIO is at the IOMMU group level, so the guest would not be able > > to easily split devices grouped together on the host between separate > > users in the guest. That could be modeled as a conventional PCI bridge > > masking the requester ID of devices in the guest such that host groups > > are mirrored as guest groups. > > I think that users would be happy with only one palo ucs VF wrapped by usnic > in the guest. I definitively need to check this point. The solution should support multiple devices though, it may just require multiple guest IOMMUs and fairly strict configuration constraints. > > There might also be more simple "punch-through" ways to do it, for > > instance what if instead of trying to make it work like it does on the > > host we invented a paravirt VFIO interface and the vfio-pv driver in the > > guest populated /dev/vfio as slightly modified passthroughs to the host > > fds. The guest OS may not even really need to be aware of the device. > > > > As I am not really interested in nesting VFIO but using the intel IOMMU > directly > in the guest a "punch-through" method would be fine. I was doing a lot of hand-waving for a vfio-pv punch-though, but I don't even have a vague idea of what an IOMMU API punch-through would look like. Seems like you need to evaluate if the pain of emulating VT-d is greater than the pain of creating a new pv IOMMU and which is likely to perform better. Thanks, Alex > > It's an interesting project and certainly a valid use case. I'd also > > like to see things like Intel's DPDK move to using VFIO, but the current > > UIO DPDK is often used in guests. Thanks, > > I ask this to Thomas Monjalon the DPDK maintainer. > > Thanks, > > Best regards > > Benoît > > > > > Alex > > > >
