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

["Christian König" <christian.koenig@amd.com>]

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>