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

["Prica, Nikola" <nikola.prica@amd.com>]

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)
>