linux-kernel - RE: [PATCH] vhost: introduce vDPA based backend

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Message-ID: <AM0PR0502MB37952015716C1D5E07E390B6C3020@AM0PR0502MB3795.eurprd05.prod.outlook.com>
Date:   Wed, 5 Feb 2020 07:15:03 +0000
From:   Shahaf Shuler <shahafs@...lanox.com>
To:     Tiwei Bie <tiwei.bie@...el.com>, Jason Wang <jasowang@...hat.com>
CC:     "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
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        Jason Gunthorpe <jgg@...lanox.com>,
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        "cunming.liang@...el.com" <cunming.liang@...el.com>,
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Subject: RE: [PATCH] vhost: introduce vDPA based backend

Wednesday, February 5, 2020 4:03 AM, Tiwei Bie:
> Subject: Re: [PATCH] vhost: introduce vDPA based backend
> 
> On Tue, Feb 04, 2020 at 11:30:11AM +0800, Jason Wang wrote:
> > On 2020/1/31 上午11:36, Tiwei Bie wrote:
> > > This patch introduces a vDPA based vhost backend. This backend is
> > > built on top of the same interface defined in virtio-vDPA and
> > > provides a generic vhost interface for userspace to accelerate the
> > > virtio devices in guest.
> > >
> > > This backend is implemented as a vDPA device driver on top of the
> > > same ops used in virtio-vDPA. It will create char device entry named
> > > vhost-vdpa/$vdpa_device_index for userspace to use. Userspace can
> > > use vhost ioctls on top of this char device to setup the backend.
> > >
> > > Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>

[...]

> > > +static long vhost_vdpa_do_dma_mapping(struct vhost_vdpa *v) {
> > > +	/* TODO: fix this */
> >
> >
> > Before trying to do this it looks to me we need the following during
> > the probe
> >
> > 1) if set_map() is not supported by the vDPA device probe the IOMMU
> > that is supported by the vDPA device
> > 2) allocate IOMMU domain
> >
> > And then:
> >
> > 3) pin pages through GUP and do proper accounting
> > 4) store GPA->HPA mapping in the umem
> > 5) generate diffs of memory table and using IOMMU API to setup the dma
> > mapping in this method
> >
> > For 1), I'm not sure parent is sufficient for to doing this or need to
> > introduce new API like iommu_device in mdev.
> 
> Agree. We may also need to introduce something like the iommu_device.
> 

Would it be better for the map/umnap logic to happen inside each device ? 
Devices that needs the IOMMU will call iommu APIs from inside the driver callback. 
Devices that has other ways to do the DMA mapping will call the proprietary APIs.