Arnd Bergmann
2022-May-18 16:39 UTC
[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 at gmail.com> wrote:> On 18.05.22 17:32, Arnd Bergmann wrote: > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh at gmail.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 at 3000 { > 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