linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Thu, 3 Jun 2021 10:09:27 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Thu, Jun 03, 2021 at 10:52:51AM +0800, Jason Wang wrote:

> Basically, we don't want to bother with pseudo KVM device like what VFIO
> did. So for simplicity, we rules out the IOMMU that can't enforce coherency
> in vhost-vDPA if the parent purely depends on the platform IOMMU:

VDPA HW cannot issue no-snoop TLPs in the first place.

virtio does not define a protocol to discover such a functionality,
nor do any virtio drivers implement the required platform specific
cache flushing to make no-snoop TLPs work.

It is fundamentally part of the virtio HW PCI API that a device vendor
cannot alter.

Basically since we already know that the virtio kernel drivers do not
call the cache flush instruction we don't need the weird KVM logic to
turn it on at all.

Enforcing no-snoop at the IOMMU here is redundant/confusing.

Jason