RE: [PATCH 3/3] vfio: Report PASID capability via VFIO_DEVICE_FEATURE ioctl

[Tian, Kevin]

> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Sent: Monday, December 11, 2023 4:08 PM
> 
> On 2023/12/7 16:47, Tian, Kevin wrote:
> >> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> >> Sent: Monday, November 27, 2023 2:39 PM
> >>
> >> +static int vfio_pci_core_feature_pasid(struct vfio_device *device, u32
> flags,
> >> +				       struct vfio_device_feature_pasid __user
> >> *arg,
> >> +				       size_t argsz)
> >> +{
> >> +	struct vfio_pci_core_device *vdev =
> >> +		container_of(device, struct vfio_pci_core_device, vdev);
> >> +	struct vfio_device_feature_pasid pasid = { 0 };
> >> +	struct pci_dev *pdev = vdev->pdev;
> >> +	u32 capabilities = 0;
> >> +	int ret;
> >> +
> >> +	/* We do not support SET of the PASID capability */
> >
> > this line alone is meaningless. Please explain the reason e.g. due to
> > no PASID capability per VF...
> 
> sure. I think the major reason is we don't allow userspace to change the
> PASID configuration. is it?

if only PF it's still possible to develop a model allowing userspace to
change.

but with VF this is not possible in concept.

> >> +	if (pdev->is_virtfn)
> >> +		pdev = pci_physfn(pdev);
> >> +
> >> +	if (!pdev->pasid_enabled)
> >> +		goto out;
> >> +
> >> +#ifdef CONFIG_PCI_PASID
> >> +	pci_read_config_dword(pdev, pdev->pasid_cap + PCI_PASID_CAP,
> >> +			      &capabilities);
> >> +#endif
> >
> > #ifdef is unnecessary. If CONFIG_PCI_PASID is false pdev->pasid_enabled
> > won't be set anyway.
> 
> it's sad that the pdev->pasid_cap is defined under #if CONFIG_PCI_PASID.
> Perhaps we can have a wrapper for it.

oh I didn't note it.

> 
> > and it should read from PCI_PASID_CTRL which indicates whether a
> > capability is actually enabled.
> 
> yes, for the EXEC and PRIV capability, needs to check if it's enabled or
> not before reporting.
> 
> >
> >> +/**
> >> + * Upon VFIO_DEVICE_FEATURE_GET, return the PASID capability for the
> >> device.
> >> + * Zero width means no support for PASID.
> >
> > also mention the encoding of this field according to PCIe spec.
> 
> yes.
> 
> > or turn it to a plain number field.
> 
> It is not exact the same as the spec since bit0 is reserved. But
> here bit0 is used as well.
> 

what is bit0 used for?