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

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Date:   Thu, 21 Oct 2021 15:58:02 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        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, Oct 21, 2021 at 02:26:00AM +0000, Tian, Kevin wrote:
> > I'll leave it to Jean to confirm. If only coherent DMA can be used in
> > the guest on other platforms, suppose VFIO should not blindly set
> > IOMMU_CACHE and in concept it should deny assigning a non-coherent
> > device since no co-ordination with guest exists today.
> 
> Jean, what's your opinion?

Yes a sanity check to prevent assigning non-coherent devices would be
good, though I'm not particularly worried about non-coherent devices. PCIe
on Arm should be coherent (according to the Base System Architecture). So
vfio-pci devices should be coherent, but vfio-platform and mdev are
case-by-case (hopefully all coherent since it concerns newer platforms).

More worrying, I thought we disabled No-Snoop for VFIO but I was wrong,
it's left enabled. On Arm I don't think userspace can perform the right
cache maintenance operations to maintain coherency with a device that
issues No-Snoop writes. Userspace can issue clean+invalidate but not
invalidate alone, so there is no equivalent to arch_sync_dma_for_cpu().
I think the worse that can happen is the device owner shooting itself in
the foot by using No-Snoop, but would it hurt to disable it?

Thanks,
Jean