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

[Bjorn Helgaas]

On Mon, Jun 01, 2020 at 03:56:55PM -0600, Alex Williamson wrote:
> On Mon, 1 Jun 2020 14:40:23 -0700
> "Raj, Ashok" <ashok.raj@xxxxxxxxx> wrote:
> 
> > 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:
> > > > â 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).
> > > > â 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.
> > > > â 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@xxxxxxxxx>
> > > > To: Joerg Roedel <joro@xxxxxxxxxx>
> > > > To: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > Cc: Darrel Goeddel <DGoeddel@xxxxxxxxxxxxxx>
> > > > Cc: Mark Scott <mscott@xxxxxxxxxxxxxx>,
> > > > Cc: Romil Sharma <rsharma@xxxxxxxxxxxxxx>
> > > > Cc: Ashok Raj <ashok.raj@xxxxxxxxx>  
> > > 
> > > 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. 
> 
> Yep, without an IOMMU all devices operate in the same IOVA space and we
> have no isolation.  We only enable ACS when an IOMMU driver requests it
> and it's only used by IOMMU code to determine IOMMU grouping of
> devices.  Thanks,

Thanks, Ashok and Alex.  I wish it were more obvious from the code,
but I am reassured.

I also added a stable tag to help get this backported.