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

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Date:   Fri, 11 Jun 2021 11:21:42 -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 6/8/2021 9:02 AM, Dave Jiang wrote:
>
> 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.
>
Hi Jason. I hacked the idxd driver to remove mdev association and use 
vfio_device directly. Ran into some issues. Specifically mdev does some 
special handling when it comes to iommu domain. When we hit 
vfio_iommu_type1_attach_group(), there's a branch in there for 
mdev_bus_type. It sets the group with mdev_group flag, which later has 
effect of special handling for iommu_attach_group. And in addition, it 
ends up switching the bus to pci_bus_type before iommu_domain_alloc() is 
called.  Do we need to provide similar type of handling for vfio_device 
that are not backed by an entire PCI device like vfio_pci? Not sure it's 
the right thing to do to attach these devices to pci_bus_type directly.