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

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Message-ID: <20210607141424.GF1002214@nvidia.com>
Date:   Mon, 7 Jun 2021 11:14:24 -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 Mon, Jun 07, 2021 at 11:18:33AM +0800, Jason Wang wrote:

> Note that no drivers call these things doesn't meant it was not
> supported by the spec.

Of course it does. If the spec doesn't define exactly when the driver
should call the cache flushes for no-snoop transactions then the
protocol doesn't support no-soop. 

no-snoop is only used in very specific sequences of operations, like
certain GPU usages, because regaining coherence on x86 is incredibly
expensive.

ie I wouldn't ever expect a NIC to use no-snoop because NIC's expect
packets to be processed by the CPU.

"non-coherent DMA" is some general euphemism that evokes images of
embedded platforms that don't have coherent DMA at all and have low
cost ways to regain coherence. This is not at all what we are talking
about here at all.

Jason