Re: [PATCH] uio: add irq control support to uio_pci_generic

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 20, 2015 at 08:33:18AM -0700, Stephen Hemminger wrote:
> On Mon, 20 Apr 2015 15:59:06 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Apr 16, 2015 at 02:21:10PM -0700, Stephen Hemminger wrote:
> > > On Thu, 16 Apr 2015 09:43:24 +0200
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > 
> > > > On Wed, Apr 15, 2015 at 09:59:34AM -0700, Stephen Hemminger wrote:
> > > > > The driver already supported INTX interrupts but had no in kernel
> > > > > function to enable and disable them.
> > > > > 
> > > > > It is possible for userspace to do this by accessing PCI config
> > > > > directly, but this racy
> > > > 
> > > > How is it racy? We have userspace using this interface,
> > > > if there's a race I want to fix it.
> > > 
> > > There is nothing to prevent two threads in user space doing 
> > > read/modify write at the same time.
> > 
> > Well that's a userspace bug then - so let's drop that
> > from commit log lest people think this fixes some
> > kernel bugs. read/modify/write to the same register
> > is at least an easy to grasp problem, creating
> > an extra interface for the same function opens up
> > the possibility that some userspace will do
> > read/modify/write from one thread with irqcontrol
> > from another thread, creating more races.
> > 
> > > The bigger issue is that DPDK needs to support multiple UIO
> > > interface types. And with current model there is no abstraction.
> > > The way to enable/disable IRQ is different depending on the UIO
> > > drivers.
> > 
> > OK compatibility with other devices might be useful, but what are the
> > other UIO drivers DPDK supports? I only found support for igb_uio so
> > far, and that doesn't seem to be upstream.
> > 
> 
> Currently, supports:
>   igb_uio, uio_pci_generic (as well as vfio)
> 
> There are additional drivers which been submitted but not accepted for Xen and HyperV
> both of which require special uio drivers.

Well vfio doesn't have irq_control, does it?  So I'd say it's best to
wait and see before we commit to a new ABI then.  You probably need to
support existing kernels anyway, if igb_uio makes it upstream,
then adding an interface that's consistent with it will make sense.

-- 
MST