linux-kernel - Re: [PATCH v6 00/20] Add VFIO mediated device support and DEV-MSI support for the idxd driver

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Message-ID: <bb39b5d4-093b-ded4-8ff1-73bbd472d905@intel.com>
Date:   Tue, 8 Jun 2021 09:02:58 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Lu, Baolu" <baolu.lu@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v6 00/20] Add VFIO mediated device support and DEV-MSI
 support for the idxd driver


On 6/7/2021 12:11 PM, Jason Gunthorpe wrote:
> On Mon, Jun 07, 2021 at 11:13:04AM -0700, Dave Jiang wrote:
>
>> So in step 1, we 'tag' the wq to be dedicated to guest usage and put the
>> hardware wq into enable state. For a dedicated mode wq, we can definitely
>> just register directly and skip the mdev step. For a shared wq mode, we can
>> have multiple mdev running on top of a single wq. So we need some way to
>> create more mdevs. We can either go with the existing established creation
>> path by mdev, or invent something custom for the driver as Jason suggested
>> to accomodate additional virtual devices for guests. We implemented the mdev
>> path originally with consideration of mdev is established and has a known
>> interface already.
> It sounds like you could just as easially have a 'create new vfio'
> file under the idxd sysfs.. Especially since you already have a bus
> and dynamic vfio specific things being created on this bus.

Will explore this and using of 'struct vfio_device' without mdev.



>
> Have you gone over this with Dan?
>
>> I think things become more complicated when we go from a dedicated wq to
>> shared wq where the relationship of wq : mdev is 1 : 1 goes to 1 : N. Also
>> needing to keep a consistent user config experience is desired, especially
>> we already have such behavior since kernel 5.6 for host usages. So we really
>> need try to avoid doing wq configuration differently just for "mdev" wqs. In
>> the case suggested above, we basically just flipped the configuration steps.
>> Mdev is first created through mdev sysfs interface. And then the device
>> paramters are configured. Where for us, we configure the device parameter
>> first, and then create the mdev. But in the end, it's still the hybrid mdev
>> setup right?
> So you don't even use mdev to configure anything? Yuk.
>
> Jason