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Date:   Thu, 5 Mar 2020 13:51:08 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     kvm@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, dev@dpdk.org, mtosatti@redhat.com,
        thomas@monjalon.net, bluca@debian.org, jerinjacobk@gmail.com,
        bruce.richardson@intel.com
Subject: Re: [PATCH v2 4/7] vfio: Introduce VFIO_DEVICE_FEATURE ioctl and
 first user
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        <20200227183407.74a5c5b4.cohuck@redhat.com>
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On Thu, 27 Feb 2020 18:34:07 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Wed, 19 Feb 2020 11:54:18 -0700
> Alex Williamson <alex.williamson@redhat.com> wrote:
> 
> > The VFIO_DEVICE_FEATURE ioctl is meant to be a general purpose, device
> > agnostic ioctl for setting, retrieving, and probing device features.
> > This implementation provides a 16-bit field for specifying a feature
> > index, where the data porition of the ioctl is determined by the
> > semantics for the given feature.  Additional flag bits indicate the
> > direction and nature of the operation; SET indicates user data is
> > provided into the device feature, GET indicates the device feature is
> > written out into user data.  The PROBE flag augments determining
> > whether the given feature is supported, and if provided, whether the
> > given operation on the feature is supported.
> > 
> > The first user of this ioctl is for setting the vfio-pci VF token,
> > where the user provides a shared secret key (UUID) on a SR-IOV PF
> > device, which users must provide when opening associated VF devices.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >  drivers/vfio/pci/vfio_pci.c |   52 +++++++++++++++++++++++++++++++++++++++++++
> >  include/uapi/linux/vfio.h   |   37 +++++++++++++++++++++++++++++++
> >  2 files changed, 89 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 8dd6ef9543ca..e4d5d26e5e71 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -1180,6 +1180,58 @@ static long vfio_pci_ioctl(void *device_data,
> >  
> >  		return vfio_pci_ioeventfd(vdev, ioeventfd.offset,
> >  					  ioeventfd.data, count, ioeventfd.fd);
> > +	} else if (cmd == VFIO_DEVICE_FEATURE) {
> > +		struct vfio_device_feature feature;
> > +		uuid_t uuid;
> > +
> > +		minsz = offsetofend(struct vfio_device_feature, flags);
> > +
> > +		if (copy_from_user(&feature, (void __user *)arg, minsz))
> > +			return -EFAULT;
> > +
> > +		if (feature.argsz < minsz)
> > +			return -EINVAL;
> > +
> > +		if (feature.flags & ~(VFIO_DEVICE_FEATURE_MASK |
> > +				      VFIO_DEVICE_FEATURE_SET |
> > +				      VFIO_DEVICE_FEATURE_GET |
> > +				      VFIO_DEVICE_FEATURE_PROBE))
> > +			return -EINVAL;  
> 
> GET|SET|PROBE is well-defined, but what about GET|SET without PROBE? Do
> we want to fence this in the generic ioctl handler part? Or is there
> any sane way to implement that (read and then write back something?)

I'd be ok with discouraging combinations of GET|SET|!PROBE generically.
I don't think there's an intuitive answer to whether it should be
applied as GET|SET or SET|GET.  If some future feature wanted an atomic
op we could add something like a test-and-set.  Thanks,

Alex

> > +
> > +		switch (feature.flags & VFIO_DEVICE_FEATURE_MASK) {
> > +		case VFIO_DEVICE_FEATURE_PCI_VF_TOKEN:
> > +			if (!vdev->vf_token)
> > +				return -ENOTTY;
> > +
> > +			/*
> > +			 * We do not support GET of the VF Token UUID as this
> > +			 * could expose the token of the previous device user.
> > +			 */
> > +			if (feature.flags & VFIO_DEVICE_FEATURE_GET)
> > +				return -EINVAL;
> > +
> > +			if (feature.flags & VFIO_DEVICE_FEATURE_PROBE)
> > +				return 0;
> > +
> > +			/* Don't SET unless told to do so */
> > +			if (!(feature.flags & VFIO_DEVICE_FEATURE_SET))
> > +				return -EINVAL;
> > +
> > +			if (feature.argsz < minsz + sizeof(uuid))
> > +				return -EINVAL;
> > +
> > +			if (copy_from_user(&uuid, (void __user *)(arg + minsz),
> > +					   sizeof(uuid)))
> > +				return -EFAULT;
> > +
> > +			mutex_lock(&vdev->vf_token->lock);
> > +			uuid_copy(&vdev->vf_token->uuid, &uuid);
> > +			mutex_unlock(&vdev->vf_token->lock);
> > +
> > +			return 0;
> > +		default:
> > +			return -ENOTTY;
> > +		}
> >  	}
> >  
> >  	return -ENOTTY;  
> (...)