Re: [PATCH V7 vfio 9/9] vfio/virtio: Introduce a vfio driver over virtio devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 14, 2023 at 11:03:49AM +0200, Yishai Hadas wrote:
> On 14/12/2023 8:38, Michael S. Tsirkin wrote:
> > On Thu, Dec 07, 2023 at 12:28:20PM +0200, Yishai Hadas wrote:
> > > Introduce a vfio driver over virtio devices to support the legacy
> > > interface functionality for VFs.
> > > 
> > > Background, from the virtio spec [1].
> > > --------------------------------------------------------------------
> > > In some systems, there is a need to support a virtio legacy driver with
> > > a device that does not directly support the legacy interface. In such
> > > scenarios, a group owner device can provide the legacy interface
> > > functionality for the group member devices. The driver of the owner
> > > device can then access the legacy interface of a member device on behalf
> > > of the legacy member device driver.
> > > 
> > > For example, with the SR-IOV group type, group members (VFs) can not
> > > present the legacy interface in an I/O BAR in BAR0 as expected by the
> > > legacy pci driver. If the legacy driver is running inside a virtual
> > > machine, the hypervisor executing the virtual machine can present a
> > > virtual device with an I/O BAR in BAR0. The hypervisor intercepts the
> > > legacy driver accesses to this I/O BAR and forwards them to the group
> > > owner device (PF) using group administration commands.
> > > --------------------------------------------------------------------
> > > 
> > > Specifically, this driver adds support for a virtio-net VF to be exposed
> > > as a transitional device to a guest driver and allows the legacy IO BAR
> > > functionality on top.
> > > 
> > > This allows a VM which uses a legacy virtio-net driver in the guest to
> > > work transparently over a VF which its driver in the host is that new
> > > driver.
> > > 
> > > The driver can be extended easily to support some other types of virtio
> > > devices (e.g virtio-blk), by adding in a few places the specific type
> > > properties as was done for virtio-net.
> > > 
> > > For now, only the virtio-net use case was tested and as such we introduce
> > > the support only for such a device.
> > > 
> > > Practically,
> > > Upon probing a VF for a virtio-net device, in case its PF supports
> > > legacy access over the virtio admin commands and the VF doesn't have BAR
> > > 0, we set some specific 'vfio_device_ops' to be able to simulate in SW a
> > > transitional device with I/O BAR in BAR 0.
> > > 
> > > The existence of the simulated I/O bar is reported later on by
> > > overwriting the VFIO_DEVICE_GET_REGION_INFO command and the device
> > > exposes itself as a transitional device by overwriting some properties
> > > upon reading its config space.
> > > 
> > > Once we report the existence of I/O BAR as BAR 0 a legacy driver in the
> > > guest may use it via read/write calls according to the virtio
> > > specification.
> > > 
> > > Any read/write towards the control parts of the BAR will be captured by
> > > the new driver and will be translated into admin commands towards the
> > > device.
> > > 
> > > Any data path read/write access (i.e. virtio driver notifications) will
> > > be forwarded to the physical BAR which its properties were supplied by
> > > the admin command VIRTIO_ADMIN_CMD_LEGACY_NOTIFY_INFO upon the
> > > probing/init flow.
> > > 
> > > With that code in place a legacy driver in the guest has the look and
> > > feel as if having a transitional device with legacy support for both its
> > > control and data path flows.
> > > 
> > > [1]
> > > https://github.com/oasis-tcs/virtio-spec/commit/03c2d32e5093ca9f2a17797242fbef88efe94b8c
> > > 
> > > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > > Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> > > ---
> > >   MAINTAINERS                      |   7 +
> > >   drivers/vfio/pci/Kconfig         |   2 +
> > >   drivers/vfio/pci/Makefile        |   2 +
> > >   drivers/vfio/pci/virtio/Kconfig  |  16 +
> > >   drivers/vfio/pci/virtio/Makefile |   4 +
> > >   drivers/vfio/pci/virtio/main.c   | 567 +++++++++++++++++++++++++++++++
> > >   6 files changed, 598 insertions(+)
> > >   create mode 100644 drivers/vfio/pci/virtio/Kconfig
> > >   create mode 100644 drivers/vfio/pci/virtio/Makefile
> > >   create mode 100644 drivers/vfio/pci/virtio/main.c
> > > 
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index 012df8ccf34e..b246b769092d 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -22872,6 +22872,13 @@ L:	kvm@vger.kernel.org
> > >   S:	Maintained
> > >   F:	drivers/vfio/pci/mlx5/
> > > +VFIO VIRTIO PCI DRIVER
> > > +M:	Yishai Hadas <yishaih@nvidia.com>
> > > +L:	kvm@vger.kernel.org
> > > +L:	virtualization@lists.linux-foundation.org
> > > +S:	Maintained
> > > +F:	drivers/vfio/pci/virtio
> > > +
> > >   VFIO PCI DEVICE SPECIFIC DRIVERS
> > >   R:	Jason Gunthorpe <jgg@nvidia.com>
> > >   R:	Yishai Hadas <yishaih@nvidia.com>
> > > diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> > > index 8125e5f37832..18c397df566d 100644
> > > --- a/drivers/vfio/pci/Kconfig
> > > +++ b/drivers/vfio/pci/Kconfig
> > > @@ -65,4 +65,6 @@ source "drivers/vfio/pci/hisilicon/Kconfig"
> > >   source "drivers/vfio/pci/pds/Kconfig"
> > > +source "drivers/vfio/pci/virtio/Kconfig"
> > > +
> > >   endmenu
> > > diff --git a/drivers/vfio/pci/Makefile b/drivers/vfio/pci/Makefile
> > > index 45167be462d8..046139a4eca5 100644
> > > --- a/drivers/vfio/pci/Makefile
> > > +++ b/drivers/vfio/pci/Makefile
> > > @@ -13,3 +13,5 @@ obj-$(CONFIG_MLX5_VFIO_PCI)           += mlx5/
> > >   obj-$(CONFIG_HISI_ACC_VFIO_PCI) += hisilicon/
> > >   obj-$(CONFIG_PDS_VFIO_PCI) += pds/
> > > +
> > > +obj-$(CONFIG_VIRTIO_VFIO_PCI) += virtio/
> > > diff --git a/drivers/vfio/pci/virtio/Kconfig b/drivers/vfio/pci/virtio/Kconfig
> > > new file mode 100644
> > > index 000000000000..3a6707639220
> > > --- /dev/null
> > > +++ b/drivers/vfio/pci/virtio/Kconfig
> > > @@ -0,0 +1,16 @@
> > > +# SPDX-License-Identifier: GPL-2.0-only
> > > +config VIRTIO_VFIO_PCI
> > > +        tristate "VFIO support for VIRTIO NET PCI devices"
> > > +        depends on VIRTIO_PCI
> > > +        select VFIO_PCI_CORE
> > > +        help
> > > +          This provides support for exposing VIRTIO NET VF devices which support
> > > +          legacy IO access, using the VFIO framework that can work with a legacy
> > > +          virtio driver in the guest.
> > > +          Based on PCIe spec, VFs do not support I/O Space; thus, VF BARs shall
> > > +          not indicate I/O Space.
> > > +          As of that this driver emulated I/O BAR in software to let a VF be
> > > +          seen as a transitional device in the guest and let it work with
> > > +          a legacy driver.
> > > +
> > > +          If you don't know what to do here, say N.
> > 
> > BTW shouldn't this driver be limited to X86? Things like lack of memory
> > barriers will make legacy virtio racy on e.g. ARM will they not?
> > And endian-ness will be broken on PPC ...
> > 
> 
> OK, if so, we can come with the below extra code.
> Makes sense ?
> 
> I'll squash it as part of V8 to the relevant patch.
> 
> diff --git a/drivers/virtio/virtio_pci_modern.c
> b/drivers/virtio/virtio_pci_modern.c
> index 37a0035f8381..b652e91b9df4 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -794,6 +794,9 @@ bool virtio_pci_admin_has_legacy_io(struct pci_dev
> *pdev)
>         struct virtio_device *virtio_dev = virtio_pci_vf_get_pf_dev(pdev);
>         struct virtio_pci_device *vp_dev;
> 
> +#ifndef CONFIG_X86
> +       return false;
> +#endif
>         if (!virtio_dev)
>                 return false;
> 
> Yishai

Isn't there going to be a bunch more dead code that compiler won't be
able to elide?

-- 
MST