From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B5CF73D411D; Tue, 6 Oct 2026 21:32:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791322330; cv=none; b=ATQAeH24Fa2+Z+b4RS7Jy7AS++qkh5DD83S8kNGpTFSi9aAFVLbzP5Iw6KQ0TYzmdsx1VkQGFkW+zs/4wO1b34ze2XQ7TjdbZjrbBshb2io8XyXnUduGovyfIWEFOTUyrMuZRl588wrIHGkkM12mWjISg8XovAoydZ2Dd3Fnib4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791322330; c=relaxed/simple; bh=bzfvYExlUeiBsAerby/OOOHUWMbab8lSYw8VKLITbAM=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition:In-Reply-To; b=Y2dVDD/Aqx800TXv+6k/L4YafQ5l5VRh5by89jjQ04+FMVVTb2GkAMZGAhBN/HQ9fsHmpK7lL3C25tKDCE3uCFQcLlZz3FxygLpdEzrw0ZINC2Nt/RYXTG20AbXMkCkHGVek61iVQCO9u8xPer6V/Afy0KiIukVcnjccFT8rPDk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mu7i09Li; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mu7i09Li" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7AC151F0089C; Tue, 6 Oct 2026 21:32:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791322326; bh=lPh972gHXCkD0mD4dAGIvo5Eox0pSnObmtanCp/CfDA=; h=Date:From:To:Cc:Subject:In-Reply-To; b=mu7i09Lim4PhqcCC67x3w+Uluo0qAKUrEbAdFHhsvwu/V1lZt/aigzR71Q7rg0MsM /ytWgZzcsJ2B/d/ZqhqjdyvErwMPeNzpxLNM/MnYYaW0tG2kndql6Axzjjt6SaD56H Fbb3bMb3hBJ5bTMKUH/sLZ6iO4yKLkDMtqNd+HJ1S4KjfRaPsdr6I2wCOt22o+g6b6 vve7fEpP8DN40APkXPjDJskv4nsDESequYUBh23pdGNG/+bhPoXhOLWdAL+mKqB4N8 c+X4Kej0Ppf/y2Zdfa6mr29PUZyBUC6h+atz46KjE9/jhZ+z2J5j5J+fTg78F8ALm6 xv2T+A+ZSOT4A== Date: Tue, 6 Oct 2026 16:32:05 -0500 From: Bjorn Helgaas To: Leon Romanovsky Cc: Bjorn Helgaas , Logan Gunthorpe , Jason Gunthorpe , "Joerg Roedel (AMD)" , Will Deacon , Robin Murphy , Christian =?utf-8?B?S8O2bmln?= , Thomas =?utf-8?Q?Hellstr=C3=B6m?= , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, iommu@lists.linux.dev, Tushar Dave , linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-rdma@vger.kernel.org, kvm@vger.kernel.org, Chaitanya Kulkarni , Greg Kroah-Hartman , Jens Axboe , Alex Williamson , Ankit Agrawal , Jonathan Corbet , Shuah Khan , Randy Dunlap , Sumit Semwal Subject: Re: [PATCH v9 05/18] PCI/P2PDMA: Document directional ACS routing Message-ID: <20261006213205.GA713622@bhelgaas> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261001-fix-p2p-acs-v4-0-v9-5-1a8e0f50ddd9@nvidia.com> On Thu, Oct 01, 2026 at 02:55:13PM +0300, Leon Romanovsky wrote: > From: Leon Romanovsky > > P2PDMA documentation describes ACS controls as path-wide, although Request > and Completion controls apply to different transaction directions and only > affect peer-versus-upstream decisions at the path divergence. > > Document the fixed client and provider roles, the divergence port checked > for each TLP direction, and the conservative handling of unreadable ACS > state. Clarify which controls disable_acs_redir changes. > > Reviewed-by: Logan Gunthorpe > Tested-by: Tushar Dave > Signed-off-by: Leon Romanovsky > --- > Documentation/admin-guide/kernel-parameters.txt | 15 +++++++++------ > Documentation/driver-api/pci/p2pdma.rst | 13 +++++++++++++ > 2 files changed, 22 insertions(+), 6 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index 68647ff4bdd2..bc83e07dd5fc 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -5291,12 +5291,15 @@ Kernel parameters > disable_acs_redir=[; ...] > Specify one or more PCI devices (in the format > specified above) separated by semicolons. > - Each device specified will have the PCI ACS > - redirect capabilities forced off which will > - allow P2P traffic between devices through > - bridges without forcing it upstream. Note: > - this removes isolation between devices and > - may put more devices in an IOMMU group. > + Each device specified will have the PCI ACS P2P > + Request Redirect, Completion Redirect, and Egress > + Control features forced off. This may allow P2P > + traffic through bridges that would otherwise be > + redirected upstream. This may allow P2P traffic > + through bridges that would otherwise be redirected > + upstream and thus this removes isolation between > + devices and may cause affected devices to share > + an IOMMU group. > config_acs= > Format: > @[; ...] > diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst > index 80f8fec9b0e9..42b18610bf7d 100644 > --- a/Documentation/driver-api/pci/p2pdma.rst > +++ b/Documentation/driver-api/pci/p2pdma.rst > @@ -15,6 +15,19 @@ then based on the ACS settings the transaction can route entirely within > the PCIe hierarchy and never reach the root port. The kernel will evaluate > the PCIe topology and always permit P2P in these well-defined cases. > > +The client remains the PCIe requester when it reads or writes provider memory. > +Where the paths diverge, the kernel therefore evaluates P2P Request Redirect > +and Egress Control on the client-side port, and P2P Completion Redirect on the > +provider-side port for completions from a read. An enabled Egress Control is > +conservatively treated as a Request redirect. I think "where the paths diverge" means the point where a port decides whether to route a TLP up through its Upstream Port (or to the RC) or back down via a sibling Downstream Port? I'm not sure p2pdma.rst includes the context to interpret "divergence" yet. I think the code comments in [4/18] might also need a little more context about divergence. IIUC, this divergence point is basically a static point where the ACS controls being applied makes a routing difference. > +Below the divergence, the route toward the other branch is already upstream, > +so those P2P redirect controls do not affect it. Redirect controls for the > +reverse transaction directions do not affect the mapping. P2P DMA is routed > +through the host bridge when either applicable port redirects. If an ACS > +Control register cannot be read, P2P DMA is rejected because the kernel cannot > +establish a usable route. > + > This evaluation assumes clients issue strictly ordered Requests carrying an > Untranslated address. Its result is not defined when clients use Relaxed > Ordering or issue ATS-translated Requests because those TLP attributes can > > -- > 2.55.0 >