Date:   Fri, 6 Mar 2020 09:21:39 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dev@...k.org" <dev@...k.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "thomas@...jalon.net" <thomas@...jalon.net>,
        "bluca@...ian.org" <bluca@...ian.org>,
        "jerinjacobk@...il.com" <jerinjacobk@...il.com>,
        "Richardson, Bruce" <bruce.richardson@...el.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "Jason Wang" <jasowang@...hat.com>
Subject: RE: [PATCH v2 0/7] vfio/pci: SR-IOV support

> From: Alex Williamson
> Sent: Friday, March 6, 2020 1:34 AM
> 
> Hi Kevin,
> 
> Sorry for the delay, I've been out on PTO...
> 
> On Tue, 25 Feb 2020 02:33:27 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
> > > From: Alex Williamson
> > > Sent: Thursday, February 20, 2020 2:54 AM
> > >
> > > Changes since v1 are primarily to patch 3/7 where the commit log is
> > > rewritten, along with option parsing and failure logging based on
> > > upstream discussions.  The primary user visible difference is that
> > > option parsing is now much more strict.  If a vf_token option is
> > > provided that cannot be used, we generate an error.  As a result of
> > > this, opening a PF with a vf_token option will serve as a mechanism of
> > > setting the vf_token.  This seems like a more user friendly API than
> > > the alternative of sometimes requiring the option (VFs in use) and
> > > sometimes rejecting it, and upholds our desire that the option is
> > > always either used or rejected.
> > >
> > > This also means that the VFIO_DEVICE_FEATURE ioctl is not the only
> > > means of setting the VF token, which might call into question whether
> > > we absolutely need this new ioctl.  Currently I'm keeping it because I
> > > can imagine use cases, for example if a hypervisor were to support
> > > SR-IOV, the PF device might be opened without consideration for a VF
> > > token and we'd require the hypservisor to close and re-open the PF in
> > > order to set a known VF token, which is impractical.
> > >
> > > Series overview (same as provided with v1):
> >
> > Thanks for doing this!
> >
> > >
> > > The synopsis of this series is that we have an ongoing desire to drive
> > > PCIe SR-IOV PFs from userspace with VFIO.  There's an immediate need
> > > for this with DPDK drivers and potentially interesting future use
> >
> > Can you provide a link to the DPDK discussion?
> 
> There's a thread here which proposed an out-of-tree driver that enables
> a parallel sr-iov enabling interface for a vfio-pci own device.
> Clearly I felt strongly about it ;)
> 
> https://patches.dpdk.org/patch/58810/
> 
> Also, documentation for making use of an Intel FPGA device with DPDK
> requires the PF bound to igb_uio to support enabling SR-IOV:
> 
> https://doc.dpdk.org/guides/bbdevs/fpga_lte_fec.html

thanks. it is useful.

> 
> > > cases in virtualization.  We've been reluctant to add this support
> > > previously due to the dependency and trust relationship between the
> > > VF device and PF driver.  Minimally the PF driver can induce a denial
> > > of service to the VF, but depending on the specific implementation,
> > > the PF driver might also be responsible for moving data between VFs
> > > or have direct access to the state of the VF, including data or state
> > > otherwise private to the VF or VF driver.
> >
> > Just a loud thinking. While the motivation of VF token sounds reasonable
> > to me, I'm curious why the same concern is not raised in other usages.
> > For example, there is no such design in virtio framework, where the
> > virtio device could also be restarted, putting in separate process (vhost-
> user),
> > and even in separate VM (virtio-vhost-user), etc. Of course the para-
> > virtualized attribute of virtio implies some degree of trust, but as you
> > mentioned many SR-IOV implementations support VF->PF communication
> > which also implies some level of trust. It's perfectly fine if VFIO just tries
> > to do better than other sub-systems, but knowing how other people
> > tackle the similar problem may make the whole picture clearer. 😊
> >
> > +Jason.
> 
> We can follow the thread with Jason, but I can't really speak to
> whether virtio needs something similar or doesn't provide enough PF
> access to be concerned.  If they need a similar solution, we can
> collaborate, but the extension we're defining here is specifically part
> of the vfio-pci ABI, so it might not be easily portable to virtio.
> 
> > > To help resolve these concerns, we introduce a VF token into the VFIO
> > > PCI ABI, which acts as a shared secret key between drivers.  The
> > > userspace PF driver is required to set the VF token to a known value
> > > and userspace VF drivers are required to provide the token to access
> > > the VF device.  If a PF driver is restarted with VF drivers in use, it
> > > must also provide the current token in order to prevent a rogue
> > > untrusted PF driver from replacing a known driver.  The degree to
> > > which this new token is considered secret is left to the userspace
> > > drivers, the kernel intentionally provides no means to retrieve the
> > > current token.
> >
> > I'm wondering whether the token idea can be used beyond SR-IOV, e.g.
> > (1) we may allow vfio user space to manage Scalable IOV in the future,
> > which faces the similar challenge between the PF and mdev; (2) the
> > token might be used as a canonical way to replace off-tree acs-override
> > workaround, say, allowing the admin to assign devices within the
> > same iommu group to different VMs which trust each other. I'm not
> > sure how much complexity will be further introduced, but it's greatly
> > appreciated if you can help think a bit and if feasible abstract some
> > logic in vfio core layer for such potential usages...
> 
> I don't see how this can be used for ACS override.  Lacking ACS, we
> must assume lack of DMA isolation, which results in our IOMMU grouping.
> If we split IOMMU groups, that implies something that doesn't exist.  A
> user can already create a process that can own the vfio group and pass
> vfio devices to other tasks, with the restriction of having a single
> DMA address space.  If there is DMA isolation, then an mdev solution
> might be better, but given the IOMMU integration of SIOV, I'm not sure
> why the devices wouldn't simply be placed in separate groups by the
> IOMMU driver.  Thanks,

You are right. I overlooked the single DMA address space limitation.

> 
> Alex
> 
> > > Note that the above token is only required for this new model where
> > > both the PF and VF devices are usable through vfio-pci.  Existing
> > > models of VFIO drivers where the PF is used without SR-IOV enabled
> > > or the VF is bound to a userspace driver with an in-kernel, host PF
> > > driver are unaffected.
> > >
> > > The latter configuration above also highlights a new inverted scenario
> > > that is now possible, a userspace PF driver with in-kernel VF drivers.
> > > I believe this is a scenario that should be allowed, but should not be
> > > enabled by default.  This series includes code to set a default
> > > driver_override for VFs sourced from a vfio-pci user owned PF, such
> > > that the VFs are also bound to vfio-pci.  This model is compatible
> > > with tools like driverctl and allows the system administrator to
> > > decide if other bindings should be enabled.  The VF token interface
> > > above exists only between vfio-pci PF and VF drivers, once a VF is
> > > bound to another driver, the administrator has effectively pronounced
> > > the device as trusted.  The vfio-pci driver will note alternate
> > > binding in dmesg for logging and debugging purposes.
> > >
> > > Please review, comment, and test.  The example QEMU implementation
> > > provided with the RFC is still current for this version.  Thanks,
> > >
> > > Alex
> > >
> > > RFC:
> > >
> https://lore.kernel.org/lkml/158085337582.9445.17682266437583505502.stg
> > > it@...li.home/
> > > v1:
> > >
> https://lore.kernel.org/lkml/158145472604.16827.15751375540102298130.st
> > > git@...li.home/
> > >
> > > ---
> > >
> > > Alex Williamson (7):
> > >       vfio: Include optional device match in vfio_device_ops callbacks
> > >       vfio/pci: Implement match ops
> > >       vfio/pci: Introduce VF token
> > >       vfio: Introduce VFIO_DEVICE_FEATURE ioctl and first user
> > >       vfio/pci: Add sriov_configure support
> > >       vfio/pci: Remove dev_fmt definition
> > >       vfio/pci: Cleanup .probe() exit paths
> > >
> > >
> > >  drivers/vfio/pci/vfio_pci.c         |  383
> > > +++++++++++++++++++++++++++++++++--
> > >  drivers/vfio/pci/vfio_pci_private.h |   10 +
> > >  drivers/vfio/vfio.c                 |   20 +-
> > >  include/linux/vfio.h                |    4
> > >  include/uapi/linux/vfio.h           |   37 +++
> > >  5 files changed, 426 insertions(+), 28 deletions(-)
> >

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