Re: [PATCH v9 17/18] PCI/P2PDMA: Evaluate the ATS path for clients with ATS enabled

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] pci/p2pdma: pcim_p2pdma_set_ats_per_mapping() silently fails for pure clients
--

commit 39432dea495f68fb0a21d0daa1f31ba24e130b39
Author: Leon Romanovsky <leon@kernel.org>

PCI/P2PDMA: Evaluate the ATS path for clients with ATS enabled

P2PDMA assumes every client issues Untranslated Requests. A device with
ATS enabled may translate any address it is handed, including a bus
address, and Translation Blocking can reject its Translated Requests on
a route the Untranslated answer called usable. This commit takes the
Address Type from its ATS Enable bit and avoids handing such a client
bus addresses unless the client declared per-mapping ATS.

> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 6a63901c65fb6..9c0943fdb3e36 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
[ ... ]
> @@ -1633,6 +1674,9 @@ pci_p2pdma_map_type_tlp(struct p2pdma_provider *provider, struct device *dev,
>  	cache_index = map_types_idx(client);
>  
>  	rcu_read_lock();
> +	/* The declaration belongs to the client, the cache to the provider. */
> +	p2pdma = rcu_dereference(client->p2pdma);
> +	per_mapping = p2pdma && p2pdma->ats_per_mapping;

[Severity: High]
Does this code handle pure P2PDMA clients correctly?

Since pure clients (devices initiating P2P transfers but not providing memory)
have no mechanism or reason to call pcim_p2pdma_init(), client->p2pdma will
be NULL for them.

This means when they call pcim_p2pdma_set_ats_per_mapping(), it silently returns
and drops the configuration:

drivers/pci/p2pdma.c:pcim_p2pdma_set_ats_per_mapping() {
    ...
	p2p = rcu_dereference_protected(pdev->p2pdma, 1);
	if (!p2p)
		/*
		 * ats_per_mapping is a performance optimization,
		 ...
		 */
		return;

	p2p->ats_per_mapping = true;
}

Because client->p2pdma is NULL, per_mapping will always evaluate to false
here in pci_p2pdma_map_type_tlp(). This leads to pci_p2pdma_client_map_type()
incorrectly overriding the map type, which could return
PCI_P2PDMA_MAP_NOT_SUPPORTED and break P2P transfers that should have been
permitted.

How should pure clients register their ATS per-mapping capability if they
do not initialize pdev->p2pdma?

>  	p2pdma = rcu_dereference(pdev->p2pdma);
>  
>  	if (p2pdma)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261001-fix-p2p-acs-v4-0-v9-0-1a8e0f50ddd9@nvidia.com?part=17