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

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Message-ID: <a65d90f2-b6c6-4230-af52-8f676b3605c5@linux.intel.com>
Date: Thu, 24 Apr 2025 11:27:07 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Tushar Dave <tdave@...dia.com>, joro@...tes.org, will@...nel.org,
 robin.murphy@....com, kevin.tian@...el.com, jgg@...dia.com,
 yi.l.liu@...el.com, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: linux-pci@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH rc] iommu: Skip PASID validation for devices without PASID
 capability

On 4/24/25 10:06, 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>
> ---
>   drivers/iommu/iommu.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 4f91a740c15f..e01df4c3e709 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -3440,7 +3440,13 @@ int iommu_attach_device_pasid(struct iommu_domain *domain,
>   
>   	mutex_lock(&group->mutex);
>   	for_each_group_device(group, device) {
> -		if (pasid >= device->dev->iommu->max_pasids) {
> +		/*
> +		 * Skip PASID validation for devices without PASID support
> +		 * (max_pasids = 0). These devices cannot issue transactions
> +		 * with PASID, so they don't affect group's PASID usage.
> +		 */
> +		if ((device->dev->iommu->max_pasids > 0) &&
> +		    (pasid >= device->dev->iommu->max_pasids)) {

What the iommu driver should do when set_dev_pasid is called for a non-
PASID device? The iommu driver has no sense of iommu group, hence it has
no knowledge about this device sharing an iommu group with another PASID
capable device.