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

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Date:	Thu, 1 Oct 2015 16:03:06 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
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 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.

- Alex

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