linux-kernel - Re: [PATCH v2 2/8] iommu/vt-d: Improve iommu_enable_pci

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Date:   Fri, 11 Nov 2022 14:59:28 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "iommu@...ts.linux.dev" <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>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/8] iommu/vt-d: Improve iommu_enable_pci_caps()

On 2022/11/11 11:45, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, November 8, 2022 3:34 PM
>>
>> The PCI subsystem triggers WARN() if a feature is repeatedly enabled.
>> This improves iommu_enable_pci_caps() to avoid unnecessary kernel
>> traces through checking and enabling. This also adds kernel messages
>> if any feature enabling results in failure. It is worth noting that
>> PRI depends on ATS. This adds a check as well.
> 
> Cannot we have a helper to check whether this device has been attached
> to any domain? If no in the blocking path then disable PCI caps. If no
> in the attaching path then enable PCI caps.
> 
> I just didn't get the point of leaving them enabled while the device can
> not do any DMA at all.

Ideally, the kernel owns the default policy (default on or off). The
upper layers are able to control it over IOMMUFD uAPI or kerneld kAPI.
I can't see the benefits of associating these features with the
existence of any domain.

The VT-d spec seems to use the same idea. The control of PASID/ATS are
placed in the device context fields, while the setting of domains are
placed in the PASID entry fields.

Best regards,
baolu