linux-kernel - Re: [dpdk-dev] [PATCH 0/2] uio

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Message-ID: <20151001163918.1bc98774@urahara>
Date:	Thu, 1 Oct 2015 16:39:18 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Avi Kivity <avi@...lladb.com>, dev@...k.org, hjk@...sjkoch.de,
	gregkh@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [dpdk-dev] [PATCH 0/2] uio_msi: device driver

On Thu, 1 Oct 2015 16:03:06 -0700
Alexander Duyck <alexander.duyck@...il.com> wrote:

> On 10/01/2015 03:00 PM, Stephen Hemminger wrote:
> > On Thu, 1 Oct 2015 12:48:36 -0700
> > Alexander Duyck <alexander.duyck@...il.com> wrote:
> >
> >> On 10/01/2015 07:57 AM, Stephen Hemminger wrote:
> >>> On Thu, 1 Oct 2015 13:59:02 +0300
> >>> Avi Kivity <avi@...lladb.com> 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.


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