Re: [PATCH] PCI: Relax ACS requirement for Intel RCiEP devices.

["Raj, Ashok" <ashok.raj@intel.com>]

On Mon, Jun 01, 2020 at 04:25:19PM -0500, Bjorn Helgaas wrote:
> On Thu, May 28, 2020 at 01:57:42PM -0700, Ashok Raj wrote:
> > All Intel platforms guarantee that all root complex implementations
> > must send transactions up to IOMMU for address translations. Hence for
> > RCiEP devices that are Vendor ID Intel, can claim exception for lack of
> > ACS support.
> > 
> > 
> > 3.16 Root-Complex Peer to Peer Considerations
> > When DMA remapping is enabled, peer-to-peer requests through the
> > Root-Complex must be handled
> > as follows:
> > • The input address in the request is translated (through first-level,
> >   second-level or nested translation) to a host physical address (HPA).
> >   The address decoding for peer addresses must be done only on the
> >   translated HPA. Hardware implementations are free to further limit
> >   peer-to-peer accesses to specific host physical address regions
> >   (or to completely disallow peer-forwarding of translated requests).
> > • Since address translation changes the contents (address field) of
> >   the PCI Express Transaction Layer Packet (TLP), for PCI Express
> >   peer-to-peer requests with ECRC, the Root-Complex hardware must use
> >   the new ECRC (re-computed with the translated address) if it
> >   decides to forward the TLP as a peer request.
> > • Root-ports, and multi-function root-complex integrated endpoints, may
> >   support additional peerto-peer control features by supporting PCI Express
> >   Access Control Services (ACS) capability. Refer to ACS capability in
> >   PCI Express specifications for details.
> > 
> > Since Linux didn't give special treatment to allow this exception, certain
> > RCiEP MFD devices are getting grouped in a single iommu group. This
> > doesn't permit a single device to be assigned to a guest for instance.
> > 
> > In one vendor system: Device 14.x were grouped in a single IOMMU group.
> > 
> > /sys/kernel/iommu_groups/5/devices/0000:00:14.0
> > /sys/kernel/iommu_groups/5/devices/0000:00:14.2
> > /sys/kernel/iommu_groups/5/devices/0000:00:14.3
> > 
> > After the patch:
> > /sys/kernel/iommu_groups/5/devices/0000:00:14.0
> > /sys/kernel/iommu_groups/5/devices/0000:00:14.2
> > /sys/kernel/iommu_groups/6/devices/0000:00:14.3 <<< new group
> > 
> > 14.0 and 14.2 are integrated devices, but legacy end points.
> > Whereas 14.3 was a PCIe compliant RCiEP.
> > 
> > 00:14.3 Network controller: Intel Corporation Device 9df0 (rev 30)
> > Capabilities: [40] Express (v2) Root Complex Integrated Endpoint, MSI 00
> > 
> > This permits assigning this device to a guest VM.
> > 
> > Fixes: f096c061f552 ("iommu: Rework iommu_group_get_for_pci_dev()")
> > Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> > To: Joerg Roedel <joro@8bytes.org>
> > To: Bjorn Helgaas <bhelgaas@google.com>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: iommu@lists.linux-foundation.org
> > Cc: Lu Baolu <baolu.lu@linux.intel.com>
> > Cc: Alex Williamson <alex.williamson@redhat.com>
> > Cc: Darrel Goeddel <DGoeddel@forcepoint.com>
> > Cc: Mark Scott <mscott@forcepoint.com>,
> > Cc: Romil Sharma <rsharma@forcepoint.com>
> > Cc: Ashok Raj <ashok.raj@intel.com>
> 
> Tentatively applied to pci/virtualization for v5.8, thanks!
> 
> The spec says this handling must apply "when DMA remapping is
> enabled".  The patch does not check whether DMA remapping is enabled.
> 
> Is there any case where DMA remapping is *not* enabled, and we rely on
> this patch to tell us whether the device is isolated?  It sounds like
> it may give the wrong answer in such a case?
> 
> Can you confirm that I don't need to worry about this?  

I think all of this makes sense only when DMA remapping is enabled.
Otherwise there is no enforcement for isolation. 

Cheers,
Ashok