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Date:	Tue, 6 Oct 2015 17:38:21 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...lladb.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Vlad Zolotarov <vladz@...udius-systems.com>,
	linux-kernel@...r.kernel.org, hjk@...sjkoch.de, corbet@....net,
	bruce.richardson@...el.com, avi@...udius-systems.com,
	gleb@...udius-systems.com, stephen@...workplumber.org,
	alexander.duyck@...il.com
Subject: Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

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.

-- 
MST
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