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

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Message-ID:
 <SI2PR01MB4393B8FFD7BB58420514E9DDDC94A@SI2PR01MB4393.apcprd01.prod.exchangelabs.com>
Date: Fri, 23 Jan 2026 09:06:06 +0800
From: Wei Wang <wei.w.wang@...mail.com>
To: Jason Gunthorpe <jgg@...dia.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 1/22/26 11:39 PM, Jason Gunthorpe wrote:
> 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..
> 

What problems did you encounter? I've prepared a new version that adds 
the enhanced ACS controls check to pci_acs_enabled() (not via 
REQ_ACS_FLAGS though). I can post it soon so we can see if the issues 
you ran into still apply.