From: Nikola Prica <nikola.prica@amd.com>

pci_enable_atomic_ops_to_root() currently fails when no Root Port is
visible. That is common in passthrough guests (ESXi, Hyper-V): the
endpoint is assigned to the VM, but the guest topology has no Root
Port above it.

In those setups the hypervisor may already have enabled AtomicOp
Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
treat AtomicOps as already enabled and return success instead of
failing the Root Port walk.

Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Nikola Prica <nikola.prica@amd.com>

---
v3: Add Fixes tag
v2: Be more strict and only apply logic in headless device case.
---
 drivers/pci/pci.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b2879a6be5f8..62729ade496f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
 	}
 
 	root = pcie_find_root_port(dev);
-	if (!root)
+	if (!root) {
+		/*
+		 * A hypervisor may expose a headless topology with no
+		 * visible root port.  If it has already set AtomicOp
+		 * Requester Enable, there is nothing more to do.
+		 */
+		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
+		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
+			return 0;
+
 		return -EINVAL;
+	}
 
 	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
 	if ((cap & cap_mask) != cap_mask)
-- 
2.34.1

On 9/18/26 11:28, Nikola Prica wrote:
> From: Nikola Prica <nikola.prica@amd.com>
> 
> pci_enable_atomic_ops_to_root() currently fails when no Root Port is
> visible. That is common in passthrough guests (ESXi, Hyper-V): the
> endpoint is assigned to the VM, but the guest topology has no Root
> Port above it.
> 
> In those setups the hypervisor may already have enabled AtomicOp
> Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
> treat AtomicOps as already enabled and return success instead of
> failing the Root Port walk.
> 
> Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
> Reviewed-by: Christian König <christian.koenig@amd.com>
> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
> Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
> Signed-off-by: Nikola Prica <nikola.prica@amd.com>

CC: stable@vger.kernel.org # 7.1+

Bjorn do you want to pick that up? Alternative I can push it through drm-misc-fixes.

Initially I thought that this is a new feature, but it turned out to be an regression and we need to get it fixed ASAP.

Thanks,
Christian.

> ---
> v3: Add Fixes tag
> v2: Be more strict and only apply logic in headless device case.
> ---
>  drivers/pci/pci.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b2879a6be5f8..62729ade496f 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
>  	}
>  
>  	root = pcie_find_root_port(dev);
> -	if (!root)
> +	if (!root) {
> +		/*
> +		 * A hypervisor may expose a headless topology with no
> +		 * visible root port.  If it has already set AtomicOp
> +		 * Requester Enable, there is nothing more to do.
> +		 */
> +		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
> +		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
> +			return 0;
> +
>  		return -EINVAL;
> +	}
>  
>  	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
>  	if ((cap & cap_mask) != cap_mask)

On Fri, Sep 18, 2026 at 02:07:46PM +0200, Christian König wrote:
> On 9/18/26 11:28, Nikola Prica wrote:
> > From: Nikola Prica <nikola.prica@amd.com>
> > 
> > pci_enable_atomic_ops_to_root() currently fails when no Root Port is
> > visible. That is common in passthrough guests (ESXi, Hyper-V): the
> > endpoint is assigned to the VM, but the guest topology has no Root
> > Port above it.
> > 
> > In those setups the hypervisor may already have enabled AtomicOp
> > Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
> > treat AtomicOps as already enabled and return success instead of
> > failing the Root Port walk.
> > 
> > Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
> > Reviewed-by: Christian König <christian.koenig@amd.com>
> > Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
> > Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
> > Signed-off-by: Nikola Prica <nikola.prica@amd.com>
> 
> CC: stable@vger.kernel.org # 7.1+
> 
> Bjorn do you want to pick that up? Alternative I can push it through
> drm-misc-fixes.
> 
> Initially I thought that this is a new feature, but it turned out to
> be an regression and we need to get it fixed ASAP.

Let me just talk through this to make sure I understand it:

Prior to 1ae8c4ce1570, pci_enable_atomic_ops_to_root() enabled
PCI_EXP_DEVCTL2_ATOMIC_REQ unless a switch port didn't support
AtomicOp routing, a switch upstream port blocked AtomicOp egress, or a
Root Port didn't support the requested sizes.

1ae8c4ce1570 added the requirement that the Root Port be visible,
which fixed an s390 case where pci_enable_atomic_ops_to_root() enabled
AtomicOps when the Root Port did not support them but was not visible
to the kernel.

IIUC the regression is on systems where the Root Port is not visible
but *does* support AtomicOps.  Prior to 1ae8c4ce1570, we would have
enabled AtomicOps in the endpoint and returned success.  After
1ae8c4ce1570, we return -EINVAL because we can't see the Root Port to
verify its support, so the driver thinks it can't use AtomicOps.

This patch fixes the regression by assuming that if we can't see the
Root Port but the endpoint already has AtomicOps enabled, the
hypervisor or host kernel that *can* see the Root Port has already
verified its AtomicOps support, so all we have to do is return success
so the driver can use them.

I'm happy to merge this for v7.3, and I think we should add something
like this to the commit log to make it clear that it's a regression
worthy of a post-merge window fix:

  After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
  supports them"), pci_enable_atomic_ops_to_root() always fails if the
  Root Port is not visible.  On systems where the Root Port is not
  visible but *does* support AtomicOps, this is a regression: prior to
  1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
  success.

Is there any problem report for the regression?  I don't see anything
at https://linux-regtracking.leemhuis.info/regzbot/mainline/, so my
guess is no.

> > ---
> > v3: Add Fixes tag
> > v2: Be more strict and only apply logic in headless device case.
> > ---
> >  drivers/pci/pci.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index b2879a6be5f8..62729ade496f 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
> >  	}
> >  
> >  	root = pcie_find_root_port(dev);
> > -	if (!root)
> > +	if (!root) {
> > +		/*
> > +		 * A hypervisor may expose a headless topology with no
> > +		 * visible root port.  If it has already set AtomicOp
> > +		 * Requester Enable, there is nothing more to do.
> > +		 */
> > +		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
> > +		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
> > +			return 0;
> > +
> >  		return -EINVAL;
> > +	}
> >  
> >  	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
> >  	if ((cap & cap_mask) != cap_mask)
> 

On 9/18/2026 7:07 PM, Bjorn Helgaas wrote:
> On Fri, Sep 18, 2026 at 02:07:46PM +0200, Christian König wrote:
>> On 9/18/26 11:28, Nikola Prica wrote:
>>> From: Nikola Prica <nikola.prica@amd.com>
>>>
>>> pci_enable_atomic_ops_to_root() currently fails when no Root Port is
>>> visible. That is common in passthrough guests (ESXi, Hyper-V): the
>>> endpoint is assigned to the VM, but the guest topology has no Root
>>> Port above it.
>>>
>>> In those setups the hypervisor may already have enabled AtomicOp
>>> Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
>>> treat AtomicOps as already enabled and return success instead of
>>> failing the Root Port walk.
>>>
>>> Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
>>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
>>> Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
>>> Signed-off-by: Nikola Prica <nikola.prica@amd.com>
>>
>> CC: stable@vger.kernel.org # 7.1+
>>
>> Bjorn do you want to pick that up? Alternative I can push it through
>> drm-misc-fixes.
>>
>> Initially I thought that this is a new feature, but it turned out to
>> be an regression and we need to get it fixed ASAP.
> 
> Let me just talk through this to make sure I understand it:
> 
> Prior to 1ae8c4ce1570, pci_enable_atomic_ops_to_root() enabled
> PCI_EXP_DEVCTL2_ATOMIC_REQ unless a switch port didn't support
> AtomicOp routing, a switch upstream port blocked AtomicOp egress, or a
> Root Port didn't support the requested sizes.
> 

To be technically correct, some hypervisors do not allow guest VM to
set the PCI_EXP_DEVCTL2_ATOMIC_REQ. So there were cases where this
set command didn't get through. But drivers relied on return value
of pci_enable_atomic_ops_to_root(). It may be good to read it back
to make sure that set went through.

> 1ae8c4ce1570 added the requirement that the Root Port be visible,
> which fixed an s390 case where pci_enable_atomic_ops_to_root() enabled
> AtomicOps when the Root Port did not support them but was not visible
> to the kernel.
> 
> IIUC the regression is on systems where the Root Port is not visible
> but *does* support AtomicOps.  Prior to 1ae8c4ce1570, we would have
> enabled AtomicOps in the endpoint and returned success.  After
> 1ae8c4ce1570, we return -EINVAL because we can't see the Root Port to
> verify its support, so the driver thinks it can't use AtomicOps.
> 
> This patch fixes the regression by assuming that if we can't see the
> Root Port but the endpoint already has AtomicOps enabled, the
> hypervisor or host kernel that *can* see the Root Port has already
> verified its AtomicOps support, so all we have to do is return success
> so the driver can use them.
> 
> I'm happy to merge this for v7.3, and I think we should add something
> like this to the commit log to make it clear that it's a regression
> worthy of a post-merge window fix:
> 
>    After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
>    supports them"), pci_enable_atomic_ops_to_root() always fails if the
>    Root Port is not visible.  On systems where the Root Port is not
>    visible but *does* support AtomicOps, this is a regression: prior to
>    1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
>    success.
> 
> Is there any problem report for the regression?  I don't see anything
> at https://linux-regtracking.leemhuis.info/regzbot/mainline/, so my
> guess is no.
> 

Yes. That is correct. I'll send new v4 patch with updated commit log
that appends your suggestion.


This patch fixes a regression introduced in v7.0-rc1-2-g1ae8c4ce1570.

#regzbot introduced: 1ae8c4ce1570
#regzbot title: Breaks atomics operations for headless passthrough devices


Hope that this will be enough to mark it as regression. If not please let
me know.

One more question though, we encountered this issue with 6.8.0-138-generic
kernel, we figured out the regression point is 6.8.0-135-generic. Since our
validation teams are using older kernels too, is it safe to assume that
fix will be backported in new release for 6.8.0-*-generic version?

Regards,
Nikola


>>> ---
>>> v3: Add Fixes tag
>>> v2: Be more strict and only apply logic in headless device case.
>>> ---
>>>   drivers/pci/pci.c | 12 +++++++++++-
>>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>> index b2879a6be5f8..62729ade496f 100644
>>> --- a/drivers/pci/pci.c
>>> +++ b/drivers/pci/pci.c
>>> @@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
>>>   	}
>>>   
>>>   	root = pcie_find_root_port(dev);
>>> -	if (!root)
>>> +	if (!root) {
>>> +		/*
>>> +		 * A hypervisor may expose a headless topology with no
>>> +		 * visible root port.  If it has already set AtomicOp
>>> +		 * Requester Enable, there is nothing more to do.
>>> +		 */
>>> +		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
>>> +		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
>>> +			return 0;
>>> +
>>>   		return -EINVAL;
>>> +	}
>>>   
>>>   	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
>>>   	if ((cap & cap_mask) != cap_mask)
>>

On 9/21/26 13:04, Prica, Nikola wrote:
> This patch fixes a regression introduced in v7.0-rc1-2-g1ae8c4ce1570.
> 
> #regzbot introduced: 1ae8c4ce1570
> #regzbot title: Breaks atomics operations for headless passthrough devices

Thanks for that

> One more question though, we encountered this issue with 6.8.0-138-generic
> kernel, we figured out the regression point is 6.8.0-135-generic. Since our
> validation teams are using older kernels too, is it safe to assume that
> fix will be backported in new release for 6.8.0-*-generic version?

To make life a bit easier for the maintainers I'll answer this: 6.8 is
EOL for years now when it comes to kernel.org aka "upstream". So only
the vendor that still maintains a kernel based on that version can
answer that question (I suspect that's Canonical).

Ciao, Thorsten

On 9/21/26 13:04, Prica, Nikola wrote:
> On 9/18/2026 7:07 PM, Bjorn Helgaas wrote:
>> On Fri, Sep 18, 2026 at 02:07:46PM +0200, Christian König wrote:
>>> On 9/18/26 11:28, Nikola Prica wrote:
>>>> From: Nikola Prica <nikola.prica@amd.com>
>>>>
>>>> pci_enable_atomic_ops_to_root() currently fails when no Root Port is
>>>> visible. That is common in passthrough guests (ESXi, Hyper-V): the
>>>> endpoint is assigned to the VM, but the guest topology has no Root
>>>> Port above it.
>>>>
>>>> In those setups the hypervisor may already have enabled AtomicOp
>>>> Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
>>>> treat AtomicOps as already enabled and return success instead of
>>>> failing the Root Port walk.
>>>>
>>>> Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
>>>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>>> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
>>>> Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
>>>> Signed-off-by: Nikola Prica <nikola.prica@amd.com>
>>>
>>> CC: stable@vger.kernel.org # 7.1+
>>>
>>> Bjorn do you want to pick that up? Alternative I can push it through
>>> drm-misc-fixes.
>>>
>>> Initially I thought that this is a new feature, but it turned out to
>>> be an regression and we need to get it fixed ASAP.
>>
>> Let me just talk through this to make sure I understand it:
>>
>> Prior to 1ae8c4ce1570, pci_enable_atomic_ops_to_root() enabled
>> PCI_EXP_DEVCTL2_ATOMIC_REQ unless a switch port didn't support
>> AtomicOp routing, a switch upstream port blocked AtomicOp egress, or a
>> Root Port didn't support the requested sizes.
>>
> 
> To be technically correct, some hypervisors do not allow guest VM to
> set the PCI_EXP_DEVCTL2_ATOMIC_REQ. So there were cases where this
> set command didn't get through. But drivers relied on return value
> of pci_enable_atomic_ops_to_root(). It may be good to read it back
> to make sure that set went through.

Well that behavior of the hypervisor is a bit questionable.

I understand why the hypervisor blocks such configuration changes, but it is essentially a recipe for trouble.

So I agree that the Linux kernel should be as defensive as possible and double check if enabling the feature really worked as expected.

>> 1ae8c4ce1570 added the requirement that the Root Port be visible,
>> which fixed an s390 case where pci_enable_atomic_ops_to_root() enabled
>> AtomicOps when the Root Port did not support them but was not visible
>> to the kernel.
>>
>> IIUC the regression is on systems where the Root Port is not visible
>> but *does* support AtomicOps.  Prior to 1ae8c4ce1570, we would have
>> enabled AtomicOps in the endpoint and returned success.  After
>> 1ae8c4ce1570, we return -EINVAL because we can't see the Root Port to
>> verify its support, so the driver thinks it can't use AtomicOps.
>>
>> This patch fixes the regression by assuming that if we can't see the
>> Root Port but the endpoint already has AtomicOps enabled, the
>> hypervisor or host kernel that *can* see the Root Port has already
>> verified its AtomicOps support, so all we have to do is return success
>> so the driver can use them.
>>
>> I'm happy to merge this for v7.3, and I think we should add something
>> like this to the commit log to make it clear that it's a regression
>> worthy of a post-merge window fix:
>>
>>    After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
>>    supports them"), pci_enable_atomic_ops_to_root() always fails if the
>>    Root Port is not visible.  On systems where the Root Port is not
>>    visible but *does* support AtomicOps, this is a regression: prior to
>>    1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
>>    success.
>>
>> Is there any problem report for the regression?  I don't see anything
>> at https://linux-regtracking.leemhuis.info/regzbot/mainline/, so my
>> guess is no.
>>
> 
> Yes. That is correct. I'll send new v4 patch with updated commit log
> that appends your suggestion.
> 
> 
> This patch fixes a regression introduced in v7.0-rc1-2-g1ae8c4ce1570.
> 
> #regzbot introduced: 1ae8c4ce1570
> #regzbot title: Breaks atomics operations for headless passthrough devices
> 
> 
> Hope that this will be enough to mark it as regression. If not please let
> me know.

I don't think we have a public visible bug report on the issue anywhere, so that should probably do it.

> One more question though, we encountered this issue with 6.8.0-138-generic
> kernel, we figured out the regression point is 6.8.0-135-generic. Since our
> validation teams are using older kernels too, is it safe to assume that
> fix will be backported in new release for 6.8.0-*-generic version?

That is what the Fixes: tag takes care of. Most likely the kernel 6.8.0-135-generic doesn't work because the offending commit was back ported as fix.

So when the Fixes tag identifies that commit the stable maintainers should backport it as well.

Regards,
Christian.

> 
> Regards,
> Nikola
> 
> 
>>>> ---
>>>> v3: Add Fixes tag
>>>> v2: Be more strict and only apply logic in headless device case.
>>>> ---
>>>>   drivers/pci/pci.c | 12 +++++++++++-
>>>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index b2879a6be5f8..62729ade496f 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
>>>>       }
>>>>         root = pcie_find_root_port(dev);
>>>> -    if (!root)
>>>> +    if (!root) {
>>>> +        /*
>>>> +         * A hypervisor may expose a headless topology with no
>>>> +         * visible root port.  If it has already set AtomicOp
>>>> +         * Requester Enable, there is nothing more to do.
>>>> +         */
>>>> +        pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
>>>> +        if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
>>>> +            return 0;
>>>> +
>>>>           return -EINVAL;
>>>> +    }
>>>>         pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
>>>>       if ((cap & cap_mask) != cap_mask)
>>>
> 

This patch fixes a regression introduced in v7.0-rc1-2-g1ae8c4ce1570.

#regzbot introduced: 1ae8c4ce1570
#regzbot title: Breaks atomics operations for headless passthrough devices

On 9/21/2026 1:04 PM, Prica, Nikola wrote:
> On 9/18/2026 7:07 PM, Bjorn Helgaas wrote:
>> On Fri, Sep 18, 2026 at 02:07:46PM +0200, Christian König wrote:
>>> On 9/18/26 11:28, Nikola Prica wrote:
>>>> From: Nikola Prica <nikola.prica@amd.com>
>>>>
>>>> pci_enable_atomic_ops_to_root() currently fails when no Root Port is
>>>> visible. That is common in passthrough guests (ESXi, Hyper-V): the
>>>> endpoint is assigned to the VM, but the guest topology has no Root
>>>> Port above it.
>>>>
>>>> In those setups the hypervisor may already have enabled AtomicOp
>>>> Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
>>>> treat AtomicOps as already enabled and return success instead of
>>>> failing the Root Port walk.
>>>>
>>>> Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
>>>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>>> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
>>>> Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
>>>> Signed-off-by: Nikola Prica <nikola.prica@amd.com>
>>>
>>> CC: stable@vger.kernel.org # 7.1+
>>>
>>> Bjorn do you want to pick that up? Alternative I can push it through
>>> drm-misc-fixes.
>>>
>>> Initially I thought that this is a new feature, but it turned out to
>>> be an regression and we need to get it fixed ASAP.
>>
>> Let me just talk through this to make sure I understand it:
>>
>> Prior to 1ae8c4ce1570, pci_enable_atomic_ops_to_root() enabled
>> PCI_EXP_DEVCTL2_ATOMIC_REQ unless a switch port didn't support
>> AtomicOp routing, a switch upstream port blocked AtomicOp egress, or a
>> Root Port didn't support the requested sizes.
>>
> 
> To be technically correct, some hypervisors do not allow guest VM to
> set the PCI_EXP_DEVCTL2_ATOMIC_REQ. So there were cases where this
> set command didn't get through. But drivers relied on return value
> of pci_enable_atomic_ops_to_root(). It may be good to read it back
> to make sure that set went through.
> 
>> 1ae8c4ce1570 added the requirement that the Root Port be visible,
>> which fixed an s390 case where pci_enable_atomic_ops_to_root() enabled
>> AtomicOps when the Root Port did not support them but was not visible
>> to the kernel.
>>
>> IIUC the regression is on systems where the Root Port is not visible
>> but *does* support AtomicOps.  Prior to 1ae8c4ce1570, we would have
>> enabled AtomicOps in the endpoint and returned success.  After
>> 1ae8c4ce1570, we return -EINVAL because we can't see the Root Port to
>> verify its support, so the driver thinks it can't use AtomicOps.
>>
>> This patch fixes the regression by assuming that if we can't see the
>> Root Port but the endpoint already has AtomicOps enabled, the
>> hypervisor or host kernel that *can* see the Root Port has already
>> verified its AtomicOps support, so all we have to do is return success
>> so the driver can use them.
>>
>> I'm happy to merge this for v7.3, and I think we should add something
>> like this to the commit log to make it clear that it's a regression
>> worthy of a post-merge window fix:
>>
>>    After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
>>    supports them"), pci_enable_atomic_ops_to_root() always fails if the
>>    Root Port is not visible.  On systems where the Root Port is not
>>    visible but *does* support AtomicOps, this is a regression: prior to
>>    1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
>>    success.
>>
>> Is there any problem report for the regression?  I don't see anything
>> at https://linux-regtracking.leemhuis.info/regzbot/mainline/, so my
>> guess is no.
>>
> 
> Yes. That is correct. I'll send new v4 patch with updated commit log
> that appends your suggestion.
> 
> 
> This patch fixes a regression introduced in v7.0-rc1-2-g1ae8c4ce1570.
> 
> #regzbot introduced: 1ae8c4ce1570
> #regzbot title: Breaks atomics operations for headless passthrough devices
> 
> 
> Hope that this will be enough to mark it as regression. If not please let
> me know.
> 
> One more question though, we encountered this issue with 6.8.0-138-generic
> kernel, we figured out the regression point is 6.8.0-135-generic. Since our
> validation teams are using older kernels too, is it safe to assume that
> fix will be backported in new release for 6.8.0-*-generic version?
> 
> Regards,
> Nikola
> 
> 
>>>> ---
>>>> v3: Add Fixes tag
>>>> v2: Be more strict and only apply logic in headless device case.
>>>> ---
>>>>   drivers/pci/pci.c | 12 +++++++++++-
>>>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index b2879a6be5f8..62729ade496f 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
>>>>       }
>>>>       root = pcie_find_root_port(dev);
>>>> -    if (!root)
>>>> +    if (!root) {
>>>> +        /*
>>>> +         * A hypervisor may expose a headless topology with no
>>>> +         * visible root port.  If it has already set AtomicOp
>>>> +         * Requester Enable, there is nothing more to do.
>>>> +         */
>>>> +        pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
>>>> +        if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
>>>> +            return 0;
>>>> +
>>>>           return -EINVAL;
>>>> +    }
>>>>       pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
>>>>       if ((cap & cap_mask) != cap_mask)
>>>
> 

From: Nikola Prica <nikola.prica@amd.com>

pci_enable_atomic_ops_to_root() currently fails when no Root Port is
visible. That is common in passthrough guests (ESXi, Hyper-V): the
endpoint is assigned to the VM, but the guest topology has no Root
Port above it.

In those setups the hypervisor may already have enabled AtomicOp
Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
treat AtomicOps as already enabled and return success instead of
failing the Root Port walk.

After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
supports them"), pci_enable_atomic_ops_to_root() always fails if the
Root Port is not visible.  On systems where the Root Port is not
visible but *does* support AtomicOps, this is a regression: prior to
1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
success.

Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Nikola Prica <nikola.prica@amd.com>

---
v4: Updates commit log to make regression point clear
v3: Add Fixes tag
v2: Be more strict and only apply logic in headless device case.
---
 drivers/pci/pci.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b2879a6be5f8..62729ade496f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
 	}
 
 	root = pcie_find_root_port(dev);
-	if (!root)
+	if (!root) {
+		/*
+		 * A hypervisor may expose a headless topology with no
+		 * visible root port.  If it has already set AtomicOp
+		 * Requester Enable, there is nothing more to do.
+		 */
+		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
+		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
+			return 0;
+
 		return -EINVAL;
+	}
 
 	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
 	if ((cap & cap_mask) != cap_mask)
-- 
2.34.1