The ACS Enhanced bits are intended to address a lack of precision in the
spec about what ACS P2P Request Redirect is supposed to do. While Linux
has long assumed that PCI_ACS_RR would cover MMIO BARs located in the root
port and PCIe Switch ports, the spec took the position that it is
implementation specific.

To get the behavior Linux has long assumed it should be setting:

  PCI_ACS_RR | PCI_ACS_DSP_MT_RR | PCI_ACS_USP_MT_RR | PCI_ACS_UNCLAMED_RR

Follow this guidance in enable_acs and set the additional bits if ACS
Enhanced is supported.

Allow config_acs to control these bits if the device has ACS Enhanced.

The spec permits the HW to wire the bits, so after setting them
pci_acs_flags_enabled() does do a pci_read_config_word() to read the
actual value in effect.

Note that currently Linux sets these bits to 0, so any new HW that comes
supporting ACS Enhanced will end up with historical Linux disabling these
functions. Devices wanting to be compatible with old Linux will need to
wire the ctrl bits to follow ACS_RR. Devices that implement ACS Enhanced
and support the ctrl=0 behavior will break PASID SVA support and VFIO
isolation when ACS_RR is enabled.

Due to the above I strongly encourage backporting this change otherwise
old kernels may have issues with new generations of PCI switches.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/pci/pci.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b0f4d98036cddd..983f71211f0055 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -957,6 +957,7 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
 			     const char *p, const u16 acs_mask, const u16 acs_flags)
 {
 	u16 flags = acs_flags;
+	u16 supported_flags;
 	u16 mask = acs_mask;
 	char *delimit;
 	int ret = 0;
@@ -1001,8 +1002,14 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
 			}
 		}
 
-		if (mask & ~(PCI_ACS_SV | PCI_ACS_TB | PCI_ACS_RR | PCI_ACS_CR |
-			    PCI_ACS_UF | PCI_ACS_EC | PCI_ACS_DT)) {
+		supported_flags = PCI_ACS_SV | PCI_ACS_TB | PCI_ACS_RR |
+				  PCI_ACS_CR | PCI_ACS_UF | PCI_ACS_EC |
+				  PCI_ACS_DT;
+		if (caps->cap & PCI_ACS_ENHANCED)
+			supported_flags |= PCI_ACS_USP_MT_RR |
+					   PCI_ACS_DSP_MT_RR |
+					   PCI_ACS_UNCLAIMED_RR;
+		if (mask & ~supported_flags) {
 			pci_err(dev, "Invalid ACS flags specified\n");
 			return;
 		}
@@ -1062,6 +1069,14 @@ static void pci_std_enable_acs(struct pci_dev *dev, struct pci_acs *caps)
 	/* Upstream Forwarding */
 	caps->ctrl |= (caps->cap & PCI_ACS_UF);
 
+	/*
+	 * USP/DSP Memory Target Access Control and Unclaimed Request Redirect
+	 */
+	if (caps->cap & PCI_ACS_ENHANCED) {
+		caps->ctrl |= PCI_ACS_USP_MT_RR | PCI_ACS_DSP_MT_RR |
+			      PCI_ACS_UNCLAIMED_RR;
+	}
+
 	/* Enable Translation Blocking for external devices and noats */
 	if (pci_ats_disabled() || dev->external_facing || dev->untrusted)
 		caps->ctrl |= (caps->cap & PCI_ACS_TB);
-- 
2.43.0