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

[Lu, Baolu]

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.

Best regards,
baolu