Re: [PATCH 1/3] xen/pci: do not register devices outside of PCI segment scope

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jan 10, 2025 at 03:01:48PM +0100, Roger Pau Monne wrote:
> The PCI segment value is limited to 16 bits, however there are buses like VMD
> that fake being part of the PCI topology by adding segment with a number
> outside the scope of the PCI firmware specification range (>= 0x10000). The
> MCFG ACPI Table "PCI Segment Group Number" field is defined as having a 16 bit
> width.
>
> Attempting to register or manage those devices with Xen would result in errors
> at best, or overlaps with existing devices living on the truncated equivalent
> segment values.

The ACPI _SEG method (ACPI r6.5, sec 6.5.6) and the corresponding
value in the MCFG table (PCI Firmware r3.3, sec 4.1.2) are clearly
16-bit values.

But otherwise, the segment value is pretty much an arbitrary software
value, and the kernel works fine with the larger domain values from
vmd_find_free_domain(), so this isn't quite enough to explain what the
issue with Xen is.

Does Xen truncate the domain to 16 bits or use it to look up something
in ACPI?

> Skip notifying Xen about those devices.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
>  drivers/xen/pci.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/xen/pci.c b/drivers/xen/pci.c
> index 416f231809cb..08e82fd1263e 100644
> --- a/drivers/xen/pci.c
> +++ b/drivers/xen/pci.c
> @@ -43,6 +43,13 @@ static int xen_add_device(struct device *dev)
>  		pci_mcfg_reserved = true;
>  	}
>  #endif
> +
> +	if (pci_domain_nr(pci_dev->bus) >> 16) {
> +		dev_info(dev,
> +			 "not registering with Xen: invalid PCI segment\n");
> +		return 0;
> +	}
> +
>  	if (pci_seg_supported) {
>  		DEFINE_RAW_FLEX(struct physdev_pci_device_add, add, optarr, 1);
>  
> @@ -149,6 +156,12 @@ static int xen_remove_device(struct device *dev)
>  	int r;
>  	struct pci_dev *pci_dev = to_pci_dev(dev);
>  
> +	if (pci_domain_nr(pci_dev->bus) >> 16) {
> +		dev_info(dev,
> +			 "not unregistering with Xen: invalid PCI segment\n");
> +		return 0;
> +	}
> +
>  	if (pci_seg_supported) {
>  		struct physdev_pci_device device = {
>  			.seg = pci_domain_nr(pci_dev->bus),
> @@ -182,6 +195,12 @@ int xen_reset_device(const struct pci_dev *dev)
>  		.flags = PCI_DEVICE_RESET_FLR,
>  	};
>  
> +	if (pci_domain_nr(dev->bus) >> 16) {
> +		dev_info(&dev->dev,
> +			 "unable to notify Xen of device reset: invalid PCI segment\n");
> +		return 0;
> +	}
> +
>  	return HYPERVISOR_physdev_op(PHYSDEVOP_pci_device_reset, &device);
>  }
>  EXPORT_SYMBOL_GPL(xen_reset_device);
> -- 
> 2.46.0
>