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

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Date:   Thu, 18 Jun 2020 15:48:05 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "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

On Wed, 17 Jun 2020 08:28:24 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > 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?

If a call into a UAPI fails, there should be nothing to undo.  Creating
a partial setup for a failed call that needs to be undone by the caller
is not good practice.

> 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?

Yes, it seems like vfio would need to make use of a driver API to do
this, we shouldn't be faking a user buffer in the kernel in order to
call through to a UAPI.  Thanks,

Alex