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

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Message-ID: <20220405141008.GS2120790@nvidia.com>
Date:   Tue, 5 Apr 2022 11:10:08 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     "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 Tue, Apr 05, 2022 at 02:12:42PM +0800, Lu Baolu wrote:
> 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?

IHMO it is not so different from how we determine if ACS like
functionality is supported on non-PCI. It is really just a more narrow
application of the existing ACS idea.

For instance it may be that if the iommu_group came from DT we can
assume it is static and then singleton can know ACS is reliable.

Jason