linux-kernel - RE: [PATCH v2 1/3] docs: IOMMU user API

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Date:   Wed, 17 Jun 2020 08:28:24 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>
Subject: RE: [PATCH v2 1/3] docs: IOMMU user API

> From: Liu, Yi L <yi.l.liu@...el.com>
> Sent: Wednesday, June 17, 2020 2:20 PM
> 
> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > Sent: Tuesday, June 16, 2020 11:22 PM
> >
> > On Thu, 11 Jun 2020 17:27:27 -0700
> > Jacob Pan <jacob.jun.pan@...ux.intel.com> wrote:
> >
> > > >
> > > > But then I thought it even better if VFIO leaves the entire
> > > > copy_from_user() to the layer consuming it.
> > > >
> > > OK. Sounds good, that was what Kevin suggested also. I just wasn't
> > > sure how much VFIO wants to inspect, I thought VFIO layer wanted to do
> > > a sanity check.
> > >
> > > Anyway, I will move copy_from_user to iommu uapi layer.
> >
> > Just one more point brought up by Yi when we discuss this offline.
> >
> > If we move copy_from_user to iommu uapi layer, then there will be
> multiple
> > copy_from_user calls for the same data when a VFIO container has
> multiple domains,
> > devices. For bind, it might be OK. But might be additional overhead for TLB
> flush
> > request from the guest.
> 
> I think it is the same with bind and TLB flush path. will be multiple
> copy_from_user.

multiple copies is possibly fine. In reality we allow only one group per
nesting container (as described in patch [03/15]), and usually there
is just one SVA-capable device per group.

> 
> BTW. for moving data copy to iommy layer, there is another point which
> need to consider. VFIO needs to do unbind in bind path if bind failed,
> so it will assemble unbind_data and pass to iommu layer. If iommu layer
> do the copy_from_user, I think it will be failed. any idea?
> 

This might be mitigated if we go back to use the same bind_data for both
bind/unbind. Then you can reuse the user object for unwinding.

However there is another case where VFIO may need to assemble the
bind_data itself. When a VM is killed, VFIO needs to walk allocated PASIDs
and unbind them one-by-one. In such case copy_from_user doesn't work
since the data is created by kernel. Alex, do you have a suggestion how this
usage can be supported? e.g. asking IOMMU driver to provide two sets of
APIs to handle user/kernel generated requests?

Thanks
Kevin