Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 21 Oct 2021 15:58:02 +0100
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "hch@lst.de" <hch@lst.de>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "parav@mellanox.com" <parav@mellanox.com>,
        "lkml@metux.net" <lkml@metux.net>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "lushenming@huawei.com" <lushenming@huawei.com>,
        "eric.auger@redhat.com" <eric.auger@redhat.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "yi.l.liu@linux.intel.com" <yi.l.liu@linux.intel.com>,
        "Tian, Jun J" <jun.j.tian@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "jacob.jun.pan@linux.intel.com" <jacob.jun.pan@linux.intel.com>,
        "kwankhede@nvidia.com" <kwankhede@nvidia.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "dwmw2@infradead.org" <dwmw2@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
        "david@gibson.dropbear.id.au" <david@gibson.dropbear.id.au>,
        "nicolinc@nvidia.com" <nicolinc@nvidia.com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO
Message-ID: <YXF/+jxRtjnlXU7w@myrica>
References: <20210922234954.GB964074@nvidia.com>
 <BN9PR11MB5433409DF766AAEF1BB2CF258CA39@BN9PR11MB5433.namprd11.prod.outlook.com>
 <BN9PR11MB54333BDB1E58387FD9999DF18CA39@BN9PR11MB5433.namprd11.prod.outlook.com>
 <20210923114219.GG964074@nvidia.com>
 <BN9PR11MB5433519229319BA951CA97638CAA9@BN9PR11MB5433.namprd11.prod.outlook.com>
 <20210930222355.GH964074@nvidia.com>
 <BN9PR11MB5433530032DC8400B71FCB788CB89@BN9PR11MB5433.namprd11.prod.outlook.com>
 <20211014154259.GT2744544@nvidia.com>
 <BN9PR11MB543327BB6D58AEF91AD2C9D18CB99@BN9PR11MB5433.namprd11.prod.outlook.com>
 <BL1PR11MB5429973588E4FBCEC8F519A88CBF9@BL1PR11MB5429.namprd11.prod.outlook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <BL1PR11MB5429973588E4FBCEC8F519A88CBF9@BL1PR11MB5429.namprd11.prod.outlook.com>
Precedence: bulk

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