linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Enable PASID during iommu device probe

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Date:   Thu, 15 Sep 2022 11:00:21 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Ethan Zhao <haifeng.zhao@...ux.intel.com>, iommu@...ts.linux.dev
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Enable PASID during iommu device probe



On 9/14/22 4:59 PM, Ethan Zhao wrote:
>>> What the error path would be if this code runs on some old platforms 
>>> don't
>>>
>>> support PASID, would you print out "this platform doesn't suppor 
>>> PASID" and
>>>
>>> give users an interface function to query if the PASID cap of iommu 
>>> is enabled
>>>
>>> and if not why ?
>>
>> It's not an error case if the IOMMU doesn't support PASID. But it's an
>> error case if any device drivers call PASID related IOMMU interfaces
>> (for example, iommu_domain_attach/detach_dev_pasid()). The corresponding
>> error code will be returned to the drivers.
>>
> So iommu driver withdraws the flexibility/rights from device driver 
> about the
> 
> ability to enable PASID, looks much more *integrated* iommu works as 
> relation

No. This patch doesn't withdraw anything. It's just a code refactoring.

> 
> controller in device-iommu-domain by enabling PASID in iommu probe stage
> 
> by default -- iommu decides to enable PASID or not though device-iommu-
> 
> domain might not work ? or they should work because they are hard-wired
> 
> together (device - iommu) even device is hotpluged later.
> 

I may not get you exactly. :-) Some IOMMU features reply on PASID
capabilities on both IOMMU and device. The IOMMU drivers enumerate the
capabilities and enable them if they are supported.

Best regards,
baolu