[PATCH 5.9 026/255] PCI: Always enable ACS even if no ACS Capability

[Greg Kroah-Hartman]

From: Rajat Jain <rajatja@xxxxxxxxxx>

[ Upstream commit 462b58fb033996e999cc213ed0b430d4f22a28fe ]

Some devices support ACS functionality even though they don't have a
spec-compliant ACS Capability; pci_enable_acs() has a quirk mechanism to
handle them.

We want to enable ACS whenever possible, but 52fbf5bdeeef ("PCI: Cache ACS
capability offset in device") inadvertently broke this by calling
pci_enable_acs() only if we find an ACS Capability.

This resulted in ACS not being enabled for these non-compliant devices,
which means devices can't be separated into different IOMMU groups, which
in turn means we may not be able to pass those devices through to VMs, as
reported by Boris V:

  https://lore.kernel.org/r/74aeea93-8a46-5f5a-343c-790d4c655da3@xxxxxxxxxx

Fixes: 52fbf5bdeeef ("PCI: Cache ACS capability offset in device")
Link: https://lore.kernel.org/r/20201028231545.4116866-1-rajatja@xxxxxxxxxx
Reported-by: Boris V <borisvk@xxxxxxxxxx>
Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Reviewed-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/pci/pci.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index e39c5499770ff..b2fed944903e2 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3503,8 +3503,13 @@ void pci_acs_init(struct pci_dev *dev)
 {
 	dev->acs_cap = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ACS);
 
-	if (dev->acs_cap)
-		pci_enable_acs(dev);
+	/*
+	 * Attempt to enable ACS regardless of capability because some Root
+	 * Ports (e.g. those quirked with *_intel_pch_acs_*) do not have
+	 * the standard ACS capability but still support ACS via those
+	 * quirks.
+	 */
+	pci_enable_acs(dev);
 }
 
 /**
-- 
2.27.0