[PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[Nikola Prica]

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.

Signed-off-by: Nikola Prica <nikola.prica@amd.com>
---
Changes in 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

Re: [PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[Christian König]

On 9/7/26 18:17, 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.
> 
> Signed-off-by: Nikola Prica <nikola.prica@amd.com>

Reviewed-by: Christian König <christian.koenig@amd.com>

Bjorn first of all any objections to this? It sounds save to me, only a handful of drivers actually use this function and it generally seems to be the right things to do.

Then if you agree any objections to up-streaming it through AMDs GPU branch? That would make things a bit easier for us.

Thanks,
Christian.

> ---
> Changes in 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)

Re: [PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[Prica, Nikola]

Hi Bjorn, any thoughs on this? Do you need more information?

Thanks,
Nikola

On 9/8/2026 10:47 AM, Christian König wrote:
> On 9/7/26 18:17, 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.
>>
>> Signed-off-by: Nikola Prica <nikola.prica@amd.com>
> 
> Reviewed-by: Christian König <christian.koenig@amd.com>
> 
> Bjorn first of all any objections to this? It sounds save to me, only a handful of drivers actually use this function and it generally seems to be the right things to do.
> 
> Then if you agree any objections to up-streaming it through AMDs GPU branch? That would make things a bit easier for us.
> 
> Thanks,
> Christian.
> 
>> ---
>> Changes in 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)
> 

Re: [PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[Gerd Bayer]

On Mon, 2026-09-07 at 18:17 +0200, 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.

Hi Nikola,

your patch got me interested, since it touches those parts of
pci_enable_atomic_ops_to_root() that commit 1ae8c4ce1570 ("PCI: Enable
AtomicOps only if Root Port supports them") has modified.

I like your solution for putting the hypervisor in control of the
enablement of Atomic Ops on root-less PCI functions and did test your
change on s390/Connect-X, successfully.

> Signed-off-by: Nikola Prica <nikola.prica@amd.com>

If that commit of mine regressed your use-cases, you might even want to
add a Fixes: tag?

> ---
> Changes in 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)

At any rate, feel free to add my

Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>

Re: [PATCH v2] PCI: Accept AtomicOps already enabled by the hypervisor

[Christian König]

On 9/14/26 15:41, Gerd Bayer wrote:
> 
> On Mon, 2026-09-07 at 18:17 +0200, 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.
> 
> Hi Nikola,
> 
> your patch got me interested, since it touches those parts of
> pci_enable_atomic_ops_to_root() that commit 1ae8c4ce1570 ("PCI: Enable
> AtomicOps only if Root Port supports them") has modified.
> 
> I like your solution for putting the hypervisor in control of the
> enablement of Atomic Ops on root-less PCI functions and did test your
> change on s390/Connect-X, successfully.
> 
>> Signed-off-by: Nikola Prica <nikola.prica@amd.com>
> 
> If that commit of mine regressed your use-cases, you might even want to
> add a Fixes: tag?

Oh, that's an interesting point. I wasn't aware that the root complex check was added so recently.

So indeed question @Nikola did that worked out of the box before kernel 7.1? If yes then that would be a regression.

Regards,
Christian.

> 
>> ---
>> Changes in 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)
> 
> At any rate, feel free to add my
> 
> Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
> Tested-by: Gerd Bayer <gbayer@linux.ibm.com>