linux-kernel - RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET

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Date:   Thu, 30 Sep 2021 08:49:03 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>, "Lu, Baolu" <baolu.lu@...el.com>
CC:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, September 23, 2021 8:22 PM
> 
> > > These are different things and need different bits. Since the ARM path
> > > has a lot more code supporting it, I'd suggest Intel should change
> > > their code to use IOMMU_BLOCK_NO_SNOOP and abandon
> IOMMU_CACHE.
> >
> > I didn't fully get this point. The end result is same, i.e. making the DMA
> > cache-coherent when IOMMU_CACHE is set. Or if you help define the
> > behavior of IOMMU_CACHE, what will you define now?
> 
> It is clearly specifying how the kernel API works:
> 
>  !IOMMU_CACHE
>    must call arch cache flushers
>  IOMMU_CACHE -
>    do not call arch cache flushers
>  IOMMU_CACHE|IOMMU_BLOCK_NO_SNOOP -
>    dot not arch cache flushers, and ignore the no snoop bit.

Who will set IOMMU_BLOCK_NO_SNOOP? I feel this is arch specific
knowledge about how cache coherency is implemented, i.e. 
when IOMMU_CACHE is set intel-iommu driver just maps it to
blocking no-snoop. It's not necessarily to be an attribute in 
the same level as IOMMU_CACHE?

> 
> On Intel it should refuse to create a !IOMMU_CACHE since the HW can't
> do that. 

Agree. In reality I guess this is not hit because all devices are marked
coherent on Intel platforms...

Baolu, any insight here?

Thanks
Kevin