Re: [PATCH v2 1/3] iommu: Make single-device group for PASID explicit

[Baolu Lu]

On 2023/8/22 16:24, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Tuesday, August 22, 2023 2:37 PM
> >
> > On 2023/8/21 14:33, Tian, Kevin wrote:
> > > > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > Sent: Monday, August 21, 2023 1:45 PM
> > > >
> > > > On 2023/8/18 11:56, Tian, Kevin wrote:
> > > > > > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > > > Sent: Monday, August 14, 2023 9:18 AM
> > > > > >
> > > > > > The PASID interfaces have always supported only single-device groups.
> > > > > > This was first introduced in commit 26b25a2b98e45 ("iommu: Bind
> > > > process
> > > > > > address spaces to devices"), and has been kept consistent in
> > subsequent
> > > > > > commits.
> > > > > >
> > > > > > However, the core code doesn't explicitly check for this requirement
> > > > > > after commit 201007ef707a8 ("PCI: Enable PASID only when ACS RR &
> > UF
> > > > > > enabled on upstream path"), which made this requirement implicit.
> > > > > >
> > > > > > Restore the check to make it explicit that the PASID interfaces only
> > > > > > support devices belonging to single-device groups.
> > > > > >
> > > > > > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >     drivers/iommu/iommu.c | 5 +++++
> > > > > >     1 file changed, 5 insertions(+)
> > > > > >
> > > > > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > > > > > index 71b9c41f2a9e..f1eba60e573f 100644
> > > > > > --- a/drivers/iommu/iommu.c
> > > > > > +++ b/drivers/iommu/iommu.c
> > > > > > @@ -3408,6 +3408,11 @@ int iommu_attach_device_pasid(struct
> > > > > > iommu_domain *domain,
> > > > > >     		return -ENODEV;
> > > > > >
> > > > > >     	mutex_lock(&group->mutex);
> > > > > > +	if (list_count_nodes(&group->devices) != 1) {
> > > > > > +		ret = -EINVAL;
> > > > > > +		goto out_unlock;
> > > > > > +	}
> > > > > > +
> > > > >
> > > > > I wonder whether we should also block adding new device to this
> > > > > group once the single-device has pasid enabled. Otherwise the
> > > >
> > > > This has been guaranteed by pci_enable_pasid():
> > > >
> > > >            if (!pci_acs_path_enabled(pdev, NULL, PCI_ACS_RR | PCI_ACS_UF))
> > > >                    return -EINVAL;
> > >
> > > well since you are adding generic core check then it's not good to
> > > rely on the fact of a specific bus...
> >
> > We attempted to do this in the patch linked below.
> >
> > https://lore.kernel.org/linux-iommu/20220705050710.2887204-5-
> > baolu.lu@xxxxxxxxxxxxxxx/
> >
> > After long discussion, we decided to move it to the pci_enable_pasid()
> > interface. The non-static single device group is only relevant to PCI
> > fabrics that support hot-plugging without ACS support on the upstream
> > path.
>
> If that's the case better add a comment to include this fact. So
> another one looking at this code won't fall into the same puzzle
> wondering what about a group becoming non-singleton after
> above check. 😊

Yeah, fair enough.

Best regards,
baolu