linux-kernel - Re: [PATCH v1] PCI: Add support for ACS Enhanced Capability

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Message-ID: <20260120164112.GD1134360@nvidia.com>
Date: Tue, 20 Jan 2026 12:41:12 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Wei Wang <wei.w.wang@...mail.com>
Cc: bhelgaas@...gle.com, akpm@...ux-foundation.org, bp@...en8.de,
	rdunlap@...radead.org, alex@...zbot.org, kevin.tian@...el.com,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v1] PCI: Add support for ACS Enhanced Capability

On Tue, Jan 20, 2026 at 02:11:30AM +0800, Wei Wang wrote:
> Add support for the ACS (Access Control Services) Enhanced Capability,
> introduced with PCIe Gen 5. These new configuration options can be
> controlled via the config_acs= boot parameter.
> 
> By default, the ACS Unclaimed Request Redirect Control (URRC) bit is
> enabled if supported by the hardware (i.e., if the ACS Enhanced Capability
> is present). This setting is particularly important for device passthrough
> in virtualization scenarios.

The memory target access bits should be set to request redirect as
well. Linux's grouping logic effectively has assumed the enabled behavior
forever.

Jason