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

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Message-ID: <20260122153953.GP1134360@nvidia.com>
Date: Thu, 22 Jan 2026 11:39:53 -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 Wed, Jan 21, 2026 at 05:18:46PM +0800, Wei Wang wrote:
> On 1/21/26 12:41 AM, Jason Gunthorpe wrote:
> > 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.
> > 
> Yes, sounds good, thanks.
> I think the REQ_ACS_FLAGS also needs to include the check.

That would make a mess, I think leave it alone. I tried to make some
patches for it once but it is going to be problematic..

Jason