linux-kernel - Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton

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Date:   Tue, 5 Apr 2022 14:12:42 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

On 2022/4/5 1:24, Jason Gunthorpe wrote:
> On Mon, Apr 04, 2022 at 01:43:49PM +0800, Lu Baolu wrote:
>> On 2022/3/30 19:58, Jason Gunthorpe wrote:
>>>>> Testing the group size is inherently the wrong test to make.
>>>> What is your suggestion then?
>>> Add a flag to the group that positively indicates the group can never
>>> have more than one member, even after hot plug. eg because it is
>>> impossible due to ACS, or lack of bridges, and so on.
>>
>> The check method seems to be bus specific. For platform devices, perhaps
>> this kind of information should be retrieved from firmware interfaces
>> like APCI or DT.
>>
>>  From this point of view, would it be simpler and more reasonable for the
>> device driver to do such check? After all, it is the device driver that
>> decides whether to provide SVA services to the application via uacce.
> 
> The check has to do with the interconnect, not the device - I don't
> see how a device driver would know any better.

I'm worried about how to support this group flag for devices that are
not connected to the system through PCI buses. If IOMMU can support
sva_bind() only when this flag is set, the SVA on many devices cannot
be supported. Or this flag is always set for non PCI devices by default?

> 
> Why do you bring up uacce? Nothing should need uacce to access SVA.

The uacce is irrelevant here.

Best regards,
baolu