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

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Message-ID: <20250506180820.GB2260709@nvidia.com>
Date: Tue, 6 May 2025 15:08:20 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Tushar Dave <tdave@...dia.com>
Cc: joro@...tes.org, will@...nel.org, robin.murphy@....com,
	kevin.tian@...el.com, yi.l.liu@...el.com, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v3 rc] iommu: Skip PASID validation for devices without
 PASID capability

On Mon, May 05, 2025 at 02:15:24PM -0700, Tushar Dave wrote:
> 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>
> ---
> 
> changes in v3:
> - addressed review comment from Vasant.
> 
>  drivers/iommu/iommu.c | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason