[PATCH 6.6 50/50] Revert "PCI: Enable ACS after configuring IOMMU for OF platforms"

Greg Kroah-Hartman
2026-04-13T16:15:39Z

From: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>

This reverts commit ec494c0260bf57a6fa3aa43a91daf7a774f8bd97 which is
commit c41e2fb67e26b04d919257875fa954aa5f6e392e upstream.

The original commit attempted to enable ACS in pci_dma_configure() prior
to IOMMU group assignment in iommu_init_device() to fix the ACS enablement
issue for OF platforms. But that assumption doesn't hold true for kernel
versions prior to v6.15, because on these older kernels,
pci_dma_configure() is called *after* iommu_init_device(). So the IOMMU
groups are already created before the ACS gets enabled. This causes the
devices that should have been split into separate groups by ACS, getting
merged into one group, thereby breaking the IOMMU isolation as reported on
the AMD machines.

So revert the offending commit to restore the IOMMU group assignment on
those affected machines. It should be noted that ACS has never really
worked on kernel versions prior to v6.15, so the revert doesn't make any
difference for OF platforms.

Reported-by: John Hancock <john@kernel.doghat.io>
Reported-by: bjorn.forsman@gmail.com
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221234
Fixes: b20b659c2c6a ("PCI: Enable ACS after configuring IOMMU for OF platforms")
Cc: Linux kernel regressions list <regressions@lists.linux.dev>
Link: https://lore.kernel.org/regressions/2c30f181-ffc6-4d63-a64e-763cf4528f48@leemhuis.info
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/pci/pci-driver.c | 8 --------
drivers/pci/pci.c | 10 +++++++++-
drivers/pci/pci.h | 1 -
3 files changed, 9 insertions(+), 10 deletions(-)

--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -1668,14 +1668,6 @@ static int pci_dma_configure(struct devi
ret = acpi_dma_configure(dev, acpi_get_dma_attr(adev));
}

- /*
- * 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(to_pci_dev(dev));
-
pci_put_host_bridge_device(bridge);

if (!ret && !driver->driver_managed_dma) {
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1046,7 +1046,7 @@ static void pci_std_enable_acs(struct pc
* pci_enable_acs - enable ACS if hardware support it
* @dev: the PCI device
*/
-void pci_enable_acs(struct pci_dev *dev)
+static void pci_enable_acs(struct pci_dev *dev)
{
if (!pci_acs_enable)
goto disable_acs_redir;
@@ -3823,6 +3823,14 @@ bool pci_acs_path_enabled(struct pci_dev
void pci_acs_init(struct pci_dev *dev)
{
dev->acs_cap = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ACS);
+
+ /*
+ * 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);
}

/**
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -557,7 +557,6 @@ static inline resource_size_t pci_resour
}

void pci_acs_init(struct pci_dev *dev);
-void pci_enable_acs(struct pci_dev *dev);
#ifdef CONFIG_PCI_QUIRKS
int pci_dev_specific_acs_enabled(struct pci_dev *dev, u16 acs_flags);
int pci_dev_specific_enable_acs(struct pci_dev *dev);