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

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

On 9/3/26 14:12, 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>
> ---
>  drivers/pci/pci.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b2879a6be5f8..7c62c7f31757 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3768,6 +3768,14 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
>  		return -EINVAL;
>  	}
>  
> +	/*
> +	 * Some hypervisors already set AtomicOp Requester Enable.  If it
> +	 * is already set, there is nothing more to do.
> +	 */
> +	pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
> +	if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
> +		return 0;
> +

I think for a bit more defensiveness we should only do this when we can't find a root port. E.g. under the if (!root) below.

Apart from that looks totally sane to me.

Regards,
Christian.

>  	root = pcie_find_root_port(dev);
>  	if (!root)
>  		return -EINVAL;
> 
> base-commit: 89a312991dc6e638a36adc43ccb91dbc25504c04