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Date:   Tue, 8 Jun 2021 09:52:01 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@metux.net>
Cc:     Jason Wang <jasowang@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Robin Murphy <robin.murphy@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        David Gibson <david@gibson.dropbear.id.au>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Gerd Hoffmann <kraxel@redhat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
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On Tue, Jun 08, 2021 at 10:54:59AM +0200, Enrico Weigelt, metux IT consult wrote:

> Maybe the device as well as the transport could announce their
> capability (which IMHO should go via the virtio protocol), and if both
> are capable, the (guest's) virtio subsys tells the driver whether it's
> usable for a specific device. Perhaps we should also have a mechanism
> to tell the device that it's actually used.

The usage should be extremely narrow, like 

"If the driver issues a GPU command with flag X then the resulting
DMAs will be no-snoop and the driver must re-establish coherency at
the right moment"

It is not a general idea, but something baked directly into the device
protocol that virtio carries.

The general notion of no-nsoop should ideally be carried in the PCIe
extended config space flag. If 0 then no-snoop should never be issued,
expected, or requested.

Jason