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

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Date:   Fri, 4 Jun 2021 08:58:05 -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 Fri, Jun 04, 2021 at 09:11:03AM +0800, Jason Wang wrote:
> > nor do any virtio drivers implement the required platform specific
> > cache flushing to make no-snoop TLPs work.
> 
> I don't get why virtio drivers needs to do that. I think DMA API should hide
> those arch/platform specific stuffs from us.

It is not arch/platform stuff. If the device uses no-snoop then a
very platform specific recovery is required in the device driver.

It is not part of the normal DMA API, it is side APIs like
flush_agp_cache() or wbinvd() that are used by GPU drivers only.

If drivers/virtio doesn't explicitly call these things it doesn't
support no-snoop - hence no VDPA device can ever use no-snoop.

Since VIRTIO_F_ACCESS_PLATFORM doesn't trigger wbinvd on x86 it has
nothing to do with no-snoop.

Jason