Re: [PATCH] PCI/P2PDMA: start with a whitelist for root complexes

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Apr 18, 2019 at 01:58:59PM +0200, Christian König wrote:
> A lot of root complexes can still do P2P even when PCI devices
> don't share a common upstream bridge.
> 
> Start adding a whitelist and allow P2P if both participants are
> attached to known good root complex.

Is there a plan for addressing this in a generic way that doesn't
require an OS modification for every new "known good root complex",
e.g., some PCIe or ACPI spec update that allows the OS to discover
this?

> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/pci/p2pdma.c | 38 +++++++++++++++++++++++++++++++++++---
>  1 file changed, 35 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index c52298d76e64..212baaa7f93b 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -274,6 +274,31 @@ static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev)
>  	seq_buf_printf(buf, "%s;", pci_name(pdev));
>  }
>  
> +/*
> + * If we can't find a common upstream bridge take a look at the root complex and
> + * compare it to a whitelist of known good hardware.
> + */
> +static bool root_complex_whitelist(struct pci_dev *dev)
> +{
> +	struct pci_host_bridge *host = pci_find_host_bridge(dev->bus);
> +	struct pci_dev *root = pci_get_slot(host->bus, PCI_DEVFN(0, 0));
> +	unsigned short vendor, device;
> +
> +	if (!root)
> +		return false;
> +
> +	vendor = root->vendor;
> +	device = root->device;
> +	pci_dev_put(root);
> +
> +	/* AMD ZEN host bridges can do peer to peer */
> +	if (vendor == PCI_VENDOR_ID_AMD && device == 0x1450)
> +		return true;
> +
> +	/* TODO: Extend that to a proper whitelist */
> +	return false;
> +}
> +
>  /*
>   * Find the distance through the nearest common upstream bridge between
>   * two PCI devices.
> @@ -317,13 +342,13 @@ static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev)
>   * In this case, a list of all infringing bridge addresses will be
>   * populated in acs_list (assuming it's non-null) for printk purposes.
>   */
> -static int upstream_bridge_distance(struct pci_dev *a,
> -				    struct pci_dev *b,
> +static int upstream_bridge_distance(struct pci_dev *provider,
> +				    struct pci_dev *client,
>  				    struct seq_buf *acs_list)
>  {
> +	struct pci_dev *a = provider, *b = client, *bb;
>  	int dist_a = 0;
>  	int dist_b = 0;
> -	struct pci_dev *bb = NULL;
>  	int acs_cnt = 0;
>  
>  	/*
> @@ -354,6 +379,13 @@ static int upstream_bridge_distance(struct pci_dev *a,
>  		dist_a++;
>  	}
>  
> +	/* Allow the connection if both devices are on a whitelisted root
> +	 * complex, but add an arbitary large value to the distance.
> +	 */
> +	if (root_complex_whitelist(provider) &&
> +	    root_complex_whitelist(client))
> +		return 0x1000 + dist_a + dist_b;
> +
>  	return -1;
>  
>  check_b_path_acs:
> -- 
> 2.17.1
>