linux-kernel - RE: [PATCH v3 rc] iommu: Skip PASID validation for devices without PASID capability

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Message-ID: <BN9PR11MB52763FACEE374AD5712E3C2A8C88A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 7 May 2025 07:34:49 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Dave, Tushar" <tdave@...dia.com>, "joro@...tes.org" <joro@...tes.org>,
	"will@...nel.org" <will@...nel.org>, "robin.murphy@....com"
	<robin.murphy@....com>, "jgg@...dia.com" <jgg@...dia.com>, "Liu, Yi L"
	<yi.l.liu@...el.com>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH v3 rc] iommu: Skip PASID validation for devices without
 PASID capability

> From: Tushar Dave <tdave@...dia.com>
> Sent: Tuesday, May 6, 2025 5:15 AM
> 
> Generally PASID support requires ACS settings that usually create
> single device groups, but there are some niche cases where we can get
> multi-device groups and still have working PASID support. The primary
> issue is that PCI switches are not required to treat PASID tagged TLPs
> specially so appropriate ACS settings are required to route all TLPs to
> the host bridge if PASID is going to work properly.
> 
> pci_enable_pasid() does check that each device that will use PASID has
> the proper ACS settings to achieve this routing.
> 
> However, no-PASID devices can be combined with PASID capable devices
> within the same topology using non-uniform ACS settings. In this case
> the no-PASID devices may not have strict route to host ACS flags and
> end up being grouped with the PASID devices.
> 
> This configuration fails to allow use of the PASID within the iommu
> core code which wrongly checks if the no-PASID device supports PASID.
> 
> Fix this by ignoring no-PASID devices during the PASID validation. They
> will never issue a PASID TLP anyhow so they can be ignored.
> 
> Fixes: c404f55c26fc ("iommu: Validate the PASID in
> iommu_attach_device_pasid()")
> Cc: stable@...r.kernel.org
> Signed-off-by: Tushar Dave <tdave@...dia.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>