Re: [Qemu-devel] Guest IOMMU and Cisco usnic

[Alex Williamson]

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
> > 
> >