Re: [RFC PATCH 3/7] vfio: add spimdev support

[Kenneth Lee]

On Thu, Aug 02, 2018 at 04:24:22AM +0000, Tian, Kevin wrote:
> Date: Thu, 2 Aug 2018 04:24:22 +0000
> From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
> To: Kenneth Lee <liguozhu@xxxxxxxxxxxxx>
> CC: Kenneth Lee <nek.in.cn@xxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>,
>  Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>, "David S . Miller"
>  <davem@xxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>, Alex Williamson
>  <alex.williamson@xxxxxxxxxx>, Hao Fang <fanghao11@xxxxxxxxxx>, Zhou Wang
>  <wangzhou1@xxxxxxxxxxxxx>, Zaibo Xu <xuzaibo@xxxxxxxxxx>, Philippe
>  Ombredanne <pombredanne@xxxxxxxx>, Greg Kroah-Hartman
>  <gregkh@xxxxxxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>,
>  "linux-doc@xxxxxxxxxxxxxxx" <linux-doc@xxxxxxxxxxxxxxx>,
>  "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>,
>  "linux-crypto@xxxxxxxxxxxxxxx" <linux-crypto@xxxxxxxxxxxxxxx>,
>  "iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx" <iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
>  "kvm@xxxxxxxxxxxxxxx" <kvm@xxxxxxxxxxxxxxx>,
>  "linux-accelerators@xxxxxxxxxxxxxxxx"
>  <linux-accelerators@xxxxxxxxxxxxxxxx>, Lu Baolu
>  <baolu.lu@xxxxxxxxxxxxxxx>, "Kumar, Sanjay K" <sanjay.k.kumar@xxxxxxxxx>,
>  "linuxarm@xxxxxxxxxx" <linuxarm@xxxxxxxxxx>
> Subject: RE: [RFC PATCH 3/7] vfio: add spimdev support
> Message-ID: <AADFC41AFE54684AB9EE6CBC0274A5D19129102C@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> 
> > From: Kenneth Lee [mailto:liguozhu@xxxxxxxxxxxxx]
> > Sent: Thursday, August 2, 2018 11:47 AM
> > 
> > >
> > > > From: Kenneth Lee
> > > > Sent: Wednesday, August 1, 2018 6:22 PM
> > > >
> > > > From: Kenneth Lee <liguozhu@xxxxxxxxxxxxx>
> > > >
> > > > SPIMDEV is "Share Parent IOMMU Mdev". It is a vfio-mdev. But differ
> > from
> > > > the general vfio-mdev:
> > > >
> > > > 1. It shares its parent's IOMMU.
> > > > 2. There is no hardware resource attached to the mdev is created. The
> > > > hardware resource (A `queue') is allocated only when the mdev is
> > > > opened.
> > >
> > > Alex has concern on doing so, as pointed out in:
> > >
> > > 	https://www.spinics.net/lists/kvm/msg172652.html
> > >
> > > resource allocation should be reserved at creation time.
> > 
> > Yes. That is why I keep telling that SPIMDEV is not for "VM", it is for "many
> > processes", it is just an access point to the process. Not a device to VM. I
> > hope
> > Alex can accept it:)
> > 
> 
> VFIO is just about assigning device resource to user space. It doesn't care
> whether it's native processes or VM using the device so far. Along the direction
> which you described, looks VFIO needs to support the configuration that
> some mdevs are used for native process only, while others can be used
> for both native and VM. I'm not sure whether there is a clean way to
> enforce it...

I had the same idea at the beginning. But finally I found that the life cycle
of the virtual device for VM and process were different. Consider you create
some mdevs for VM use, you will give all those mdevs to lib-virt, which
distribute those mdev to VMs or containers. If the VM or container exits, the
mdev is returned to the lib-virt and used for next allocation. It is the
administrator who controlled every mdev's allocation.

But for process, it is different. There is no lib-virt in control. The
administrator's intension is to grant some type of application to access the
hardware. The application can get a handle of the hardware, send request and get
the result. That's all. He/She dose not care which mdev is allocated to that
application. If it crashes, it should be the kernel's responsibility to withdraw
the resource, the system administrator does not want to do it by hand.

> 
> Let's hear from Alex's thought.

Sure:)

> 
> Thanks
> Kevin

-- 
			-Kenneth(Hisilicon)

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