Re: [PATCH v5 4/7] PCI/ATS: Add PRI support for PCIe VF devices

[Kuppuswamy Sathyanarayanan]

On Mon, Aug 19, 2019 at 09:15:00AM -0500, Bjorn Helgaas wrote:
> On Thu, Aug 15, 2019 at 03:39:03PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > On 8/15/19 3:20 PM, Bjorn Helgaas wrote:
> > > [+cc Joerg, David, iommu list: because IOMMU drivers are the only
> > > callers of pci_enable_pri() and pci_enable_pasid()]
> > > 
> > > On Thu, Aug 01, 2019 at 05:06:01PM -0700, sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx wrote:
> > > > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > > > 
> > > > When IOMMU tries to enable Page Request Interface (PRI) for VF device
> > > > in iommu_enable_dev_iotlb(), it always fails because PRI support for
> > > > PCIe VF device is currently broken. Current implementation expects
> > > > the given PCIe device (PF & VF) to implement PRI capability before
> > > > enabling the PRI support. But this assumption is incorrect. As per PCIe
> > > > spec r4.0, sec 9.3.7.11, all VFs associated with PF can only use the
> > > > PRI of the PF and not implement it. Hence we need to create exception
> > > > for handling the PRI support for PCIe VF device.
> > > > 
> > > > Also, since PRI is a shared resource between PF/VF, following rules
> > > > should apply.
> > > > 
> > > > 1. Use proper locking before accessing/modifying PF resources in VF
> > > >     PRI enable/disable call.
> > > > 2. Use reference count logic to track the usage of PRI resource.
> > > > 3. Disable PRI only if the PRI reference count (pri_ref_cnt) is zero.
> 
> > > Wait, why do we need this at all?  I agree the spec says VFs may not
> > > implement PRI or PASID capabilities and that VFs use the PRI and
> > > PASID of the PF.
> > > 
> > > But why do we need to support pci_enable_pri() and pci_enable_pasid()
> > > for VFs?  There's nothing interesting we can *do* in the VF, and
> > > passing it off to the PF adds all this locking mess.  For VFs, can we
> > > just make them do nothing or return -EINVAL?  What functionality would
> > > we be missing if we did that?
> > 
> > Currently PRI/PASID capabilities are not enabled by default. IOMMU can
> > enable PRI/PASID for VF first (and not enable it for PF). In this case,
> > doing nothing for VF device will break the functionality.
> 
> What is the path where we can enable PRI/PASID for VF but not for the
> PF?  The call chains leading to pci_enable_pri() go through the
> iommu_ops.add_device interface, which makes me think this is part of
> the device enumeration done by the PCI core, and in that case I would
> think this it should be done for the PF before VFs.  But maybe this
> path isn't exercised until a driver does a DMA map or something
> similar?
AFAIK, this path will only get exercised when the device does DMA and
hence there is no specific order in which PRI/PASID is enabled in PF/VF.
In fact, my v2 version of this patch set had a check to ensure PF
PRI/PASID enable is happened before VF attempts PRI/PASID
enable/disable. But I had to remove it in later version of this series
due to failure case reported by one the tester of this code. 
> 
> Bjorn

-- 
-- 
Sathyanarayanan Kuppuswamy
Linux kernel developer