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

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Date:   Thu, 30 Sep 2021 19:08:22 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Lu, Baolu" <baolu.lu@...el.com>,
        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

On Thu, Sep 30, 2021 at 08:49:03AM +0000, Tian, Kevin wrote:
> > 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?

Basically only qemu due to specialized x86 hypervisor knowledge.

The only purpose of this attribute is to support a specific
virtualization use case where a whole bunch of stuff is broken
together:
 - the cache maintenance instructions are not available to a guest
 - the guest isn't aware that the instructions don't work and tells
   the device to issue no-snoop TLPs
 - The device ignores the 'disable no-snoop' flag in the PCIe config
   space

Thus things become broken.

Jason