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@xxxxxxx>
> >
> > 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@xxxxxxx>
>
> Reviewed-by: Christian König <christian.koenig@xxxxxxx>
>
> 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)