linux-kernel - Re: [PATCH] uio: add irq control support to uio_pci

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Date:	Mon, 20 Apr 2015 08:33:18 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Hans J. Koch" <hjk@...sjkoch.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uio: add irq control support to uio_pci_generic

On Mon, 20 Apr 2015 15:59:06 +0200
"Michael S. Tsirkin" <mst@...hat.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@...hat.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.

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