linux-kernel - Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

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Message-ID: <CAK8P3a1YhkEZ8gcbXHEa5Bwx-4VVRJO8SUHf8=RNWRsc2Yo-+A@mail.gmail.com>
Date:   Wed, 18 May 2022 17:39:23 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Oleksandr <olekstysh@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@...ts.linux-foundation.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Jason Wang <jasowang@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Julien Grall <julien@....org>, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops

On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@...il.com> wrote:
> On 18.05.22 17:32, Arnd Bergmann wrote:
> > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@...il.com> wrote:
>
> >   This would mean having a device
> > node for the grant-table mechanism that can be referred to using the 'iommus'
> > phandle property, with the domid as an additional argument.
>
> I assume, you are speaking about something like the following?
>
>
> xen_dummy_iommu {
>     compatible = "xen,dummy-iommu";
>     #iommu-cells = <1>;
> };
>
> virtio@...0 {
>     compatible = "virtio,mmio";
>     reg = <0x3000 0x100>;
>     interrupts = <41>;
>
>     /* The device is located in Xen domain with ID 1 */
>     iommus = <&xen_dummy_iommu 1>;
> };

Right, that's that's the idea, except I would not call it a 'dummy'.
>From the perspective of the DT, this behaves just like an IOMMU,
even if the exact mechanism is different from most hardware IOMMU
implementations.

> > It does not quite fit the model that Linux currently uses for iommus,
> > as that has an allocator for dma_addr_t space
>
> yes (# 3/7 adds grant-table based allocator)
>
>
> > , but it would think it's
> > conceptually close enough that it makes sense for the binding.
>
> Interesting idea. I am wondering, do we need an extra actions for this
> to work in Linux guest (dummy IOMMU driver, etc)?

It depends on how closely the guest implementation can be made to
resemble a normal iommu. If you do allocate dma_addr_t addresses,
it may actually be close enough that you can just turn the grant-table
code into a normal iommu driver and change nothing else.

        Arnd