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

[Yi Liu]

On 2023/12/12 10:20, Tian, Kevin wrote:
> > 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.

got it.

> > > > +	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.

If Alex feels better to have a wrapper, we may have one.

> > > 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?

it's just been reserved. No usage is mentioned in the latest spec. I don't
know the background neither.

--
Regards,
Yi Liu