[REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[John Hancock]

Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
platforms") introduced a regression affecting AMD IOMMU group isolation
on x86 systems, making PCIe passthrough non-functional.

While the commit addresses a legitimate ordering issue on OF/Device Tree
platforms, the fix modifies pci_dma_configure(), which executes on all
platforms regardless of firmware interface. On AMD systems with IOMMU,
moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters
the point at which ACS is evaluated relative to IOMMU group assignment.
The result is that devices which previously occupied individual, exclusive
IOMMU groups are merged into a single group containing both passthrough
and non-passthrough members, violating IOMMU isolation requirements.

The commit author notes that pci_enable_acs() is now called twice per
device and that this is "presumably not an issue." On AMD IOMMU hardware
this assumption does not hold -- the change in call ordering has
observable and breaking consequences for group topology.

It is worth noting that this is a stable/LTS series (6.12.y), where
changes to fundamental PCI initialization ordering carry significant
risk for production and specialized workloads that depend on stable
IOMMU behavior across kernel updates. A regression of this nature --
silently breaking PCIe passthrough without any configuration change on
the part of the user -- is particularly disruptive in a series that
users reasonably expect to be conservative.

This revert restores pci_enable_acs() to pci_acs_init() and marks it
static again, fully restoring correct IOMMU group topology on affected
hardware.

Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied

Hardware:
  CPU:   AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
  IOMMU: AMD-Vi

Bisect:
  6.12.74: GOOD -- IOMMU groups correct, passthrough functional
  6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
  6.12.76: BAD  -- still broken
  6.12.77: BAD  -- still broken

Fixes: 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
Signed-off-by: John Hancock <john@kernel.doghat.io>
---
 drivers/pci/pci-driver.c | 8 --------
 drivers/pci/pci.c        | 8 ++++----
 drivers/pci/pci.h        | 1 -
 3 files changed, 4 insertions(+), 13 deletions(-)

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 9846ab70c..a00a2ce01 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -1656,14 +1656,6 @@ static int pci_dma_configure(struct device *dev)
 		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) {
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 040c0be2d..3eb878781 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1072,7 +1072,7 @@ static void pci_std_enable_acs(struct pci_dev *dev, struct pci_acs *caps)
  * 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)
 {
 	struct pci_acs caps;
 	bool enable_acs = false;
@@ -3718,6 +3718,14 @@ bool pci_acs_path_enabled(struct pci_dev *start,
 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);
 }
 
 /**
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index b9b97ec0f..b1f393a42 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -653,7 +653,6 @@ static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,
 }
 
 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_cfg);
 int pci_dev_specific_enable_acs(struct pci_dev *dev);

Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[bjorn.forsman]

Hi,

Thanks for submitting this fix/revert -- I spent a lot of time today
figuring out why my VM didn't start anymore, and finally came here.

Seeing that it's been a few days since this was posted, I figured I'd
add this message as a "ping" to the committers, in hopes of getting
this merged soon. (This regression is hitting end users now.)

Best regards,
Bjørn Forsman

Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[Manivannan Sadhasivam]

+ Robin

On Fri, Mar 20, 2026 at 01:23:35PM -0400, John Hancock wrote:
> Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
> platforms") introduced a regression affecting AMD IOMMU group isolation
> on x86 systems, making PCIe passthrough non-functional.
> 
> While the commit addresses a legitimate ordering issue on OF/Device Tree
> platforms, the fix modifies pci_dma_configure(), which executes on all
> platforms regardless of firmware interface. On AMD systems with IOMMU,
> moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters
> the point at which ACS is evaluated relative to IOMMU group assignment.
> The result is that devices which previously occupied individual, exclusive
> IOMMU groups are merged into a single group containing both passthrough
> and non-passthrough members, violating IOMMU isolation requirements.
> 

Ouch! Sorry for the breakage.

> The commit author notes that pci_enable_acs() is now called twice per
> device and that this is "presumably not an issue." On AMD IOMMU hardware
> this assumption does not hold -- the change in call ordering has
> observable and breaking consequences for group topology.
> 
> It is worth noting that this is a stable/LTS series (6.12.y), where
> changes to fundamental PCI initialization ordering carry significant
> risk for production and specialized workloads that depend on stable
> IOMMU behavior across kernel updates. A regression of this nature --
> silently breaking PCIe passthrough without any configuration change on
> the part of the user -- is particularly disruptive in a series that
> users reasonably expect to be conservative.
> 

I still haven't investigated this failure deeply, but it is also worth noting
that this regression only happens with v6.12 and earlier stable kernels as
mentioned in [1].

> This revert restores pci_enable_acs() to pci_acs_init() and marks it
> static again, fully restoring correct IOMMU group topology on affected
> hardware.
> 
> Regression introduced in: 6.12.75
> Tested on: 6.12.77 with this revert applied
> 
> Hardware:
>   CPU:   AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
>   IOMMU: AMD-Vi
> 
> Bisect:
>   6.12.74: GOOD -- IOMMU groups correct, passthrough functional
>   6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
>   6.12.76: BAD  -- still broken
>   6.12.77: BAD  -- still broken
> 
> Fixes: 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
> Signed-off-by: John Hancock <john@kernel.doghat.io>

Acked-by: Manivannan Sadhasivam <mani@kernel.org>

- Mani

[1] https://lore.kernel.org/all/2c30f181-ffc6-4d63-a64e-763cf4528f48@leemhuis.info/

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Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[Robin Murphy]

On 23/03/2026 1:54 pm, Manivannan Sadhasivam wrote:
> + Robin
> 
> On Fri, Mar 20, 2026 at 01:23:35PM -0400, John Hancock wrote:
>> Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
>> platforms") introduced a regression affecting AMD IOMMU group isolation
>> on x86 systems, making PCIe passthrough non-functional.
>>
>> While the commit addresses a legitimate ordering issue on OF/Device Tree
>> platforms, the fix modifies pci_dma_configure(), which executes on all
>> platforms regardless of firmware interface. On AMD systems with IOMMU,
>> moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters
>> the point at which ACS is evaluated relative to IOMMU group assignment.
>> The result is that devices which previously occupied individual, exclusive
>> IOMMU groups are merged into a single group containing both passthrough
>> and non-passthrough members, violating IOMMU isolation requirements.
>>
> 
> Ouch! Sorry for the breakage.
> 
>> The commit author notes that pci_enable_acs() is now called twice per
>> device and that this is "presumably not an issue." On AMD IOMMU hardware
>> this assumption does not hold -- the change in call ordering has
>> observable and breaking consequences for group topology.
>>
>> It is worth noting that this is a stable/LTS series (6.12.y), where
>> changes to fundamental PCI initialization ordering carry significant
>> risk for production and specialized workloads that depend on stable
>> IOMMU behavior across kernel updates. A regression of this nature --
>> silently breaking PCIe passthrough without any configuration change on
>> the part of the user -- is particularly disruptive in a series that
>> users reasonably expect to be conservative.
>>
> 
> I still haven't investigated this failure deeply, but it is also worth noting
> that this regression only happens with v6.12 and earlier stable kernels as
> mentioned in [1].

Oops, indeed, relying on pci_dma_configure() to be called prior to group 
assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: 
Get DT/ACPI parsing into the proper probe path") added that call path in 
6.15 - thus the backport probably doesn't actually work for OF platforms 
either.

Dropping this from 6.12.y and earlier stable branches seems like the 
correct action to me (but not a mainline revert, obviously). ACS had 
essentially *never* worked properly on OF platforms prior to 6.15, but 
that was more down to fundamental design flaws in the OF-based IOMMU 
probe path (dating back to 4.12) rather than any easily-fixable bug as 
such, so realistically I think we just leave it that way.

Thanks,
Robin.

>> This revert restores pci_enable_acs() to pci_acs_init() and marks it
>> static again, fully restoring correct IOMMU group topology on affected
>> hardware.
>>
>> Regression introduced in: 6.12.75
>> Tested on: 6.12.77 with this revert applied
>>
>> Hardware:
>>    CPU:   AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
>>    IOMMU: AMD-Vi
>>
>> Bisect:
>>    6.12.74: GOOD -- IOMMU groups correct, passthrough functional
>>    6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
>>    6.12.76: BAD  -- still broken
>>    6.12.77: BAD  -- still broken
>>
>> Fixes: 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
>> Signed-off-by: John Hancock <john@kernel.doghat.io>
> 
> Acked-by: Manivannan Sadhasivam <mani@kernel.org>
> 
> - Mani
> 
> [1] https://lore.kernel.org/all/2c30f181-ffc6-4d63-a64e-763cf4528f48@leemhuis.info/
> 

Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[Manivannan Sadhasivam]

On Mon, Mar 23, 2026 at 03:06:16PM +0000, Robin Murphy wrote:
> On 23/03/2026 1:54 pm, Manivannan Sadhasivam wrote:
> > + Robin
> > 
> > On Fri, Mar 20, 2026 at 01:23:35PM -0400, John Hancock wrote:
> > > Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
> > > platforms") introduced a regression affecting AMD IOMMU group isolation
> > > on x86 systems, making PCIe passthrough non-functional.
> > > 
> > > While the commit addresses a legitimate ordering issue on OF/Device Tree
> > > platforms, the fix modifies pci_dma_configure(), which executes on all
> > > platforms regardless of firmware interface. On AMD systems with IOMMU,
> > > moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters
> > > the point at which ACS is evaluated relative to IOMMU group assignment.
> > > The result is that devices which previously occupied individual, exclusive
> > > IOMMU groups are merged into a single group containing both passthrough
> > > and non-passthrough members, violating IOMMU isolation requirements.
> > > 
> > 
> > Ouch! Sorry for the breakage.
> > 
> > > The commit author notes that pci_enable_acs() is now called twice per
> > > device and that this is "presumably not an issue." On AMD IOMMU hardware
> > > this assumption does not hold -- the change in call ordering has
> > > observable and breaking consequences for group topology.
> > > 
> > > It is worth noting that this is a stable/LTS series (6.12.y), where
> > > changes to fundamental PCI initialization ordering carry significant
> > > risk for production and specialized workloads that depend on stable
> > > IOMMU behavior across kernel updates. A regression of this nature --
> > > silently breaking PCIe passthrough without any configuration change on
> > > the part of the user -- is particularly disruptive in a series that
> > > users reasonably expect to be conservative.
> > > 
> > 
> > I still haven't investigated this failure deeply, but it is also worth noting
> > that this regression only happens with v6.12 and earlier stable kernels as
> > mentioned in [1].
> 
> Oops, indeed, relying on pci_dma_configure() to be called prior to group
> assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: Get
> DT/ACPI parsing into the proper probe path") added that call path in 6.15 -
> thus the backport probably doesn't actually work for OF platforms either.
> 

Ah, that makes sense. Thanks for finding the root cause. It might be very
obvious to you, but still... ;)

> Dropping this from 6.12.y and earlier stable branches seems like the correct
> action to me (but not a mainline revert, obviously). ACS had essentially
> *never* worked properly on OF platforms prior to 6.15, but that was more
> down to fundamental design flaws in the OF-based IOMMU probe path (dating
> back to 4.12) rather than any easily-fixable bug as such, so realistically I
> think we just leave it that way.
> 

That's my opinion as well. I guess I need to send reverts for rest of the older
stable kernels as well.

Thanks once again, Robin!

- Mani

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Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[Akemi Yagi]

On Mon, 23 Mar 2026 20:43:55 +0530, Manivannan Sadhasivam wrote:

>> Dropping this from 6.12.y and earlier stable branches seems like the
>> correct action to me (but not a mainline revert, obviously). ACS had
>> essentially *never* worked properly on OF platforms prior to 6.15, but
>> that was more down to fundamental design flaws in the OF-based IOMMU
>> probe path (dating back to 4.12) rather than any easily-fixable bug as
>> such, so realistically I think we just leave it that way.
>> 
> That's my opinion as well. I guess I need to send reverts for rest of
> the older stable kernels as well.

This is a short note to say that the 6.1 kernel was affected and the 
issue was fixed by reverting the referenced commit ( https://elrepo.org/
bugs/view.php?id=1587 ).

Relevant bugzilla entry is here:

https://bugzilla.kernel.org/show_bug.cgi?id=221234

Akemi

Re: [REGRESSION] PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

[Manivannan Sadhasivam]

+ Jason

On Mon, Mar 23, 2026 at 07:24:30PM +0530, Manivannan Sadhasivam wrote:
> + Robin
> 
> On Fri, Mar 20, 2026 at 01:23:35PM -0400, John Hancock wrote:
> > Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
> > platforms") introduced a regression affecting AMD IOMMU group isolation
> > on x86 systems, making PCIe passthrough non-functional.
> > 
> > While the commit addresses a legitimate ordering issue on OF/Device Tree
> > platforms, the fix modifies pci_dma_configure(), which executes on all
> > platforms regardless of firmware interface. On AMD systems with IOMMU,
> > moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters
> > the point at which ACS is evaluated relative to IOMMU group assignment.
> > The result is that devices which previously occupied individual, exclusive
> > IOMMU groups are merged into a single group containing both passthrough
> > and non-passthrough members, violating IOMMU isolation requirements.
> > 
> 
> Ouch! Sorry for the breakage.
> 
> > The commit author notes that pci_enable_acs() is now called twice per
> > device and that this is "presumably not an issue." On AMD IOMMU hardware
> > this assumption does not hold -- the change in call ordering has
> > observable and breaking consequences for group topology.
> > 
> > It is worth noting that this is a stable/LTS series (6.12.y), where
> > changes to fundamental PCI initialization ordering carry significant
> > risk for production and specialized workloads that depend on stable
> > IOMMU behavior across kernel updates. A regression of this nature --
> > silently breaking PCIe passthrough without any configuration change on
> > the part of the user -- is particularly disruptive in a series that
> > users reasonably expect to be conservative.
> > 
> 
> I still haven't investigated this failure deeply, but it is also worth noting
> that this regression only happens with v6.12 and earlier stable kernels as
> mentioned in [1].
> 
> > This revert restores pci_enable_acs() to pci_acs_init() and marks it
> > static again, fully restoring correct IOMMU group topology on affected
> > hardware.
> > 
> > Regression introduced in: 6.12.75
> > Tested on: 6.12.77 with this revert applied
> > 
> > Hardware:
> >   CPU:   AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
> >   IOMMU: AMD-Vi
> > 
> > Bisect:
> >   6.12.74: GOOD -- IOMMU groups correct, passthrough functional
> >   6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
> >   6.12.76: BAD  -- still broken
> >   6.12.77: BAD  -- still broken
> > 
> > Fixes: 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
> > Signed-off-by: John Hancock <john@kernel.doghat.io>
> 
> Acked-by: Manivannan Sadhasivam <mani@kernel.org>
> 
> - Mani
> 
> [1] https://lore.kernel.org/all/2c30f181-ffc6-4d63-a64e-763cf4528f48@leemhuis.info/
> 
> -- 
> மணிவண்ணன் சதாசிவம்

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