Re: [dpdk-dev] [PATCH 0/2] uio_msi: device driver

[Alexander Duyck]

On 10/01/2015 04:39 PM, Stephen Hemminger wrote:
> On Thu, 1 Oct 2015 16:03:06 -0700
> Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:
>
> > On 10/01/2015 03:00 PM, Stephen Hemminger wrote:
> > > On Thu, 1 Oct 2015 12:48:36 -0700
> > > Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:
> > >
> > > > On 10/01/2015 07:57 AM, Stephen Hemminger wrote:
> > > > > On Thu, 1 Oct 2015 13:59:02 +0300
> > > > > Avi Kivity <avi@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > > On 10/01/2015 01:28 AM, Stephen Hemminger wrote:
> > > > > > > This is a new UIO device driver to allow supporting MSI-X and MSI devices
> > > > > > > in userspace.  It has been used in environments like VMware and older versions
> > > > > > > of QEMU/KVM where no IOMMU support is available.
> > > > > > Why not add msi/msix support to uio_pci_generic?
> > > > > That is possible but that would meet ABI and other resistance from the author.
> > > > > Also, uio_pci_generic makes it harder to find resources since it doesn't fully
> > > > > utilize UIO infrastructure.
> > > > I'd say you are better off actually taking this in the other direction.
> > > >   From what I have seen it seems like this driver is meant to deal with
> > > > mapping VFs contained inside of guests.  If you are going to fork off
> > > > and create a UIO driver for mapping VFs why not just make it specialize
> > > > in that.  You could probably simplify the code by dropping support for
> > > > legacy interrupts and IO regions since all that is already covered by
> > > > uio_pci_generic anyway if I am not mistaken.
> > > >
> > > > You could then look at naming it something like uio_vf since the uio_msi
> > > > is a bit of a misnomer since it is MSI-X it supports, not MSI interrupts.
> > > The support needs to cover:
> > >     - VF in guest
> > >     - VNIC in guest (vmxnet3)
> > > it isn't just about VF's
> > I get that, but the driver you are talking about adding is duplicating
> > much of what is already there in uio_pci_generic.  If nothing else it
> > might be worth while to look at replacing the legacy interrupt with
> > MSI.  Maybe look at naming it something like uio_pcie to indicate that
> > we are focusing on assigning PCIe and virtual devices that support MSI
> > and MSI-X and use memory BARs rather than legacy PCI devices that are
> > doing things like mapping I/O BARs and using INTx signaling.
> >
> > My main argument is that we should probably look at dropping support for
> > anything that isn't going to be needed.  If it is really important we
> > can always add it later.  I just don't see the value in having code
> > around for things we aren't likely to ever use with real devices as we
> > are stuck supporting it for the life of the driver. I'll go ahead and
> > provide a inline review of your patch 2/2 as I think my feedback might
> > make a bit more sense that way.
> Ok, but having one driver that can deal with failures with msi-x vector
> setup and fallback seemed like a better strategy.

Yes, but in the case of something like a VF it is going to just make a 
bigger mess of things since INTx doesn't work.  So what would you expect 
your driver to do in that case?  Also we have to keep in mind that the 
MSI-X failure case is very unlikely.

One other thing that just occurred to me is that you may want to try 
using the range allocation call instead of a hard set number of 
interrupts.  Then if you start running short on vectors you don't hard 
fail and instead just allocate what you can.

- Alex
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