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

[Gerd Bayer <gbayer@linux.ibm.com>]

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>