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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 26 May 2020 15:17:35 -0700
Ashok Raj <ashok.raj@intel.com> 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>
> ---
>  drivers/iommu/iommu.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 2b471419e26c..31b595dfedde 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1187,7 +1187,18 @@ static struct iommu_group *get_pci_function_alias_group(struct pci_dev *pdev,
>  	struct pci_dev *tmp = NULL;
>  	struct iommu_group *group;
>  
> -	if (!pdev->multifunction || pci_acs_enabled(pdev, REQ_ACS_FLAGS))
> +	/*
> +	 * Intel VT-d Specification Section 3.16, Root-Complex Peer to Peer
> +	 * Considerations manadate that all transactions in RCiEP's and
> +	 * even Integrated MFD's *must* be sent up to the IOMMU. P2P is
> +	 * only possible on translated addresses. This gives enough
> +	 * guarantee that such devices can be forgiven for lack of ACS
> +	 * support.
> +	 */
> +	if (!pdev->multifunction ||
> +	    (pdev->vendor == PCI_VENDOR_ID_INTEL &&
> +	     pci_pcie_type(pdev) == PCI_EXP_TYPE_RC_END) ||
> +	     pci_acs_enabled(pdev, REQ_ACS_FLAGS))
>  		return NULL;
>  
>  	for_each_pci_dev(tmp) {

Hi Ashok,

As this is an Intel/VT-d standard, not a PCIe standard, why not
implement this in pci_dev_specific_acs_enabled() with all the other
quirks?  Thanks,

Alex