Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

[Kenneth Lee]

On Fri, Sep 14, 2018 at 06:50:55AM +0000, Tian, Kevin wrote:
> Date: Fri, 14 Sep 2018 06:50:55 +0000
> From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
> To: Jerome Glisse <jglisse@xxxxxxxxxx>, Kenneth Lee <liguozhu@xxxxxxxxxxxxx>
> CC: Kenneth Lee <nek.in.cn@xxxxxxxxx>, Alex Williamson
>  <alex.williamson@xxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>,
>  "kvm@xxxxxxxxxxxxxxx" <kvm@xxxxxxxxxxxxxxx>, Jonathan Corbet
>  <corbet@xxxxxxx>, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>, Zaibo
>  Xu <xuzaibo@xxxxxxxxxx>, "linux-doc@xxxxxxxxxxxxxxx"
>  <linux-doc@xxxxxxxxxxxxxxx>, "Kumar, Sanjay K" <sanjay.k.kumar@xxxxxxxxx>,
>  Hao Fang <fanghao11@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx"
>  <linux-kernel@xxxxxxxxxxxxxxx>, "linuxarm@xxxxxxxxxx"
>  <linuxarm@xxxxxxxxxx>, "iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx"
>  <iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx>, "David S . Miller"
>  <davem@xxxxxxxxxxxxx>, "linux-crypto@xxxxxxxxxxxxxxx"
>  <linux-crypto@xxxxxxxxxxxxxxx>, Zhou Wang <wangzhou1@xxxxxxxxxxxxx>,
>  Philippe Ombredanne <pombredanne@xxxxxxxx>, Thomas Gleixner
>  <tglx@xxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>,
>  "linux-accelerators@xxxxxxxxxxxxxxxx"
>  <linux-accelerators@xxxxxxxxxxxxxxxx>, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Subject: RE: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive
> Message-ID: <AADFC41AFE54684AB9EE6CBC0274A5D191303A7F@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> 
> > From: Jerome Glisse
> > Sent: Thursday, September 13, 2018 10:52 PM
> >
> [...]
>  > AFAIK, on x86 and PPC at least, all PCIE devices are in the same group
> > by default at boot or at least all devices behind the same bridge.
> 
> the group thing reflects physical hierarchy limitation, not changed
> cross boot. Please note iommu group defines the minimal isolation
> boundary - all devices within same group must be attached to the
> same iommu domain or address space, because physically IOMMU
> cannot differentiate DMAs out of those devices. devices behind
> legacy PCI-X bridge is one example. other examples include devices
> behind a PCIe switch port which doesn't support ACS thus cannot
> route p2p transaction to IOMMU. If talking about typical PCIe 
> endpoint (with upstreaming ports all supporting ACS), you'll get
> one device per group.
> 
> One iommu group today is attached to only one iommu domain.
> In the future one group may attach to multiple domains, as the
> aux domain concept being discussed in another thread.
> 
> > 
> > Maybe they are kernel option to avoid that and userspace init program
> > can definitly re-arrange that base on sysadmin policy).
> 
> I don't think there is such option, as it may break isolation model
> enabled by IOMMU.
> 
> [...]
> > > > That is why i am being pedantic :) on making sure there is good reasons
> > > > to do what you do inside VFIO. I do believe that we want a common
> > frame-
> > > > work like the one you are proposing but i do not believe it should be
> > > > part of VFIO given the baggages it comes with and that are not relevant
> > > > to the use cases for this kind of devices.
> > >
> 
> The purpose of VFIO is clear - the kernel portal for granting generic 
> device resource (mmio, irq, etc.) to user space. VFIO doesn't care
> what exactly a resource is used for (queue, cmd reg, etc.). If really
> pursuing VFIO path is necessary, maybe such common framework
> should lay down in user space, which gets all granted resource from
> kernel driver thru VFIO and then provides accelerator services to 
> other processes?

Yes. I think this is exactly what WarpDrive is now doing. This patch is just let
the type1 driver use parent IOMMU for mdev.

> 
> Thanks
> Kevin

-- 
			-Kenneth(Hisilicon)

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