Re: [PATCH v1 0/2] vfio/pci: expose device's PASID capability to VMs

[Yan Zhao]

On Tue, Mar 31, 2020 at 03:08:25PM +0800, Lu, Baolu wrote:
> On 2020/3/31 14:35, Tian, Kevin wrote:
> >> From: Liu, Yi L<yi.l.liu@xxxxxxxxx>
> >> Sent: Sunday, March 22, 2020 8:33 PM
> >>
> >> From: Liu Yi L<yi.l.liu@xxxxxxxxx>
> >>
> >> Shared Virtual Addressing (SVA), a.k.a, Shared Virtual Memory (SVM) on
> >> Intel platforms allows address space sharing between device DMA and
> >> applications. SVA can reduce programming complexity and enhance security.
> >>
> >> To enable SVA, device needs to have PASID capability, which is a key
> >> capability for SVA. This patchset exposes the device's PASID capability
> >> to guest instead of hiding it from guest.
> >>
> >> The second patch emulates PASID capability for VFs (Virtual Function) since
> >> VFs don't implement such capability per PCIe spec. This patch emulates such
> >> capability and expose to VM if the capability is enabled in PF (Physical
> >> Function).
> >>
> >> However, there is an open for PASID emulation. If PF driver disables PASID
> >> capability at runtime, then it may be an issue. e.g. PF should not disable
> >> PASID capability if there is guest using this capability on any VF related
> >> to this PF. To solve it, may need to introduce a generic communication
> >> framework between vfio-pci driver and PF drivers. Please feel free to give
> >> your suggestions on it.
> > I'm not sure how this is addressed on bate metal today, i.e. between normal
> > kernel PF and VF drivers. I look at pasid enable/disable code in intel-iommu.c.
> > There is no check on PF/VF dependency so far. The cap is toggled when
> > attaching/detaching the PF to its domain. Let's see how IOMMU guys
> > respond, and if there is a way for VF driver to block PF driver from disabling
> > the pasid cap when it's being actively used by VF driver, then we may
> > leverage the same trick in VFIO when emulation is provided to guest.
> 
> IOMMU subsystem doesn't expose any APIs for pasid enabling/disabling.
> The PCI subsystem does. It handles VF/PF like below.
> 
> /**
>   * pci_enable_pasid - Enable the PASID capability
>   * @pdev: PCI device structure
>   * @features: Features to enable
>   *
>   * Returns 0 on success, negative value on error. This function checks
>   * whether the features are actually supported by the device and returns
>   * an error if not.
>   */
> int pci_enable_pasid(struct pci_dev *pdev, int features)
> {
>          u16 control, supported;
>          int pasid = pdev->pasid_cap;
> 
>          /*
>           * VFs must not implement the PASID Capability, but if a PF
>           * supports PASID, its VFs share the PF PASID configuration.
>           */
>          if (pdev->is_virtfn) {
>                  if (pci_physfn(pdev)->pasid_enabled)
>                          return 0;
>                  return -EINVAL;
>          }
> 
> /**
>   * pci_disable_pasid - Disable the PASID capability
>   * @pdev: PCI device structure
>   */
> void pci_disable_pasid(struct pci_dev *pdev)
> {
>          u16 control = 0;
>          int pasid = pdev->pasid_cap;
> 
>          /* VFs share the PF PASID configuration */
>          if (pdev->is_virtfn)
>                  return;
> 
> 
> It doesn't block disabling PASID on PF even VFs are possibly using it.
>
hi
I'm not sure, but is it possible for pci_enable_pasid() and
pci_disable_pasid() to do the same thing as pdev->driver->sriov_configure,
e.g. pci_sriov_configure_simple() below.

It checks whether there are VFs are assigned in pci_vfs_assigned(dev).
and we can set the VF in assigned status if vfio_pci_open() is performed
on the VF.


int pci_sriov_configure_simple(struct pci_dev *dev, int nr_virtfn)
{
        int rc;

        might_sleep();

        if (!dev->is_physfn)
                return -ENODEV;

        if (pci_vfs_assigned(dev)) {
                pci_warn(dev, "Cannot modify SR-IOV while VFs are assigned\n");
                return -EPERM;
        }

        if (nr_virtfn == 0) {
                sriov_disable(dev);
                return 0;
        }

        rc = sriov_enable(dev, nr_virtfn);
        if (rc < 0)
                return rc;

        return nr_virtfn;
}

Thanks
Yan