Date:	Thu, 1 Oct 2015 16:43:23 -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 04:39 PM, Stephen Hemminger wrote:
> 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.

Yes, but in the case of something like a VF it is going to just make a 
bigger mess of things since INTx doesn't work.  So what would you expect 
your driver to do in that case?  Also we have to keep in mind that the 
MSI-X failure case is very unlikely.

One other thing that just occurred to me is that you may want to try 
using the range allocation call instead of a hard set number of 
interrupts.  Then if you start running short on vectors you don't hard 
fail and instead just allocate what you can.

- Alex
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