Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

[Vlad Zolotarov]

On 10/06/15 17:38, Michael S. Tsirkin wrote:
> On Mon, Oct 05, 2015 at 01:20:11PM +0300, Avi Kivity wrote:
> > On 10/05/2015 12:49 PM, Greg KH wrote:
> > > On Mon, Oct 05, 2015 at 11:28:03AM +0300, Avi Kivity wrote:
> > > > Of course it has to be documented, but this just follows vfio.
> > > >
> > > > Eventfd is a natural enough representation of an interrupt; both kvm and
> > > > vfio use it, and are also able to share the eventfd, allowing a vfio
> > > > interrupt to generate a kvm interrupt, without userspace intervention, and
> > > > one day without even kernel intervention.
> > > That's nice and wonderful, but it's not how UIO works today, so this is
> > > now going to be a mix and match type interface, with no justification so
> > > far as to why to create this new api and exactly how this is all going
> > > to be used from userspace.
> > The intended user is dpdk (http://dpdk.org), which is a family of userspace
> > networking drivers for high performance networking applications.
> >
> > The natural device driver for dpdk is vfio, which both provides memory
> > protection and exposes msi/msix interrupts.  However, in many cases vfio
> > cannot be used, either due to the lack of an iommu (for example, in
> > virtualized environments) or out of a desire to avoid the iommus performance
> > impact.
> >
> > The challenge in exposing msix interrupts to user space is that there are
> > many of them, so you can't simply poll the device fd.  If you do, how do you
> > know which interrupt was triggered?  The solution that vfio adopted was to
> > associate each interrupt with an eventfd, allowing it to be individually
> > polled.  Since you can pass an eventfd with SCM_RIGHTS, and since kvm can
> > trigger guest interrupts using an eventfd, the solution is very flexible.
> >
> > > Example code would be even better...
> >
> > This is the vfio dpdk interface code:
> >
> > http://dpdk.org/browse/dpdk/tree/lib/librte_eal/linuxapp/eal/eal_pci_vfio.c
> >
> > basically, the equivalent uio msix code would be very similar if uio adopts
> > a similar interface:
> >
> > http://dpdk.org/browse/dpdk/tree/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> >
> > (current code lacks msi/msix support, of course).
> So you really want a driver that behaves exactly like vfio.
> Which immediately begs a question: why not extend vfio
> to cover your usecase.

The only "like VFIO" behavior we implement here is binding the MSI-X 
interrupt notification to eventfd descriptor. This doesn't justifies the 
hassle of implementing IOMMU-less VFIO mode.



>

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