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

[Oleksandr <olekstysh@gmail.com>]

On 19.05.22 04:06, Stefano Stabellini wrote:


Hello Stefano

> On Thu, 19 May 2022, Oleksandr wrote:
>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>> <olekstysh@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@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,
>> thank you for the confirmation
>>
>>
>>
>>>    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.
>> well, agree
>>
>>
>>>>> 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.
>> Unfortunately, I failed to find a way how use grant references at the
>> iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I am
>> not too familiar with that, so what is written below might be wrong or at
>> least not precise.
>>
>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by itself, it
>> just maps (IOVA-PA) what was requested to be mapped by the upper layer. The
>> DMA address allocation is done by the upper layer (DMA-IOMMU which is the glue
>> layer between DMA API and IOMMU API allocates IOVA for PA?). But, all what we
>> need here is just to allocate our specific grant-table based DMA addresses
>> (DMA address = grant reference + offset in the page), so let’s say we need an
>> entity to take a physical address as parameter and return a DMA address (what
>> actually commit #3/7 is doing), and that’s all. So working at the dma_ops
>> layer we get exactly what we need, with the minimal changes to guest
>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>
>> Assuming that we want to reuse the IOMMU infrastructure somehow for our needs.
>> I think, in that case we will likely need to introduce a new specific IOVA
>> allocator (alongside with a generic one) to be hooked up by the DMA-IOMMU
>> layer if we run on top of Xen. But, even having the specific IOVA allocator to
>> return what we indeed need (DMA address = grant reference + offset in the
>> page) we will still need the specific minimal required IOMMU driver to be
>> present in the system anyway in order to track the mappings(?) and do nothing
>> with them, returning a success (this specific IOMMU driver should have all
>> mandatory callbacks implemented).
>>
>> I completely agree, it would be really nice to reuse generic IOMMU bindings
>> rather than introducing Xen specific property if what we are trying to
>> implement in current patch series fits in the usage of "iommus" in Linux
>> more-less. But, if we will have to add more complexity/more components to the
>> code for the sake of reusing device tree binding, this raises a question
>> whether that’s worthwhile.
>>
>> Or I really missed something?
> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> bindings, not necessarily the IOMMU drivers framework in Linux (although
> that would be an added bonus.)
>
> I know from previous discussions with you that making the grant table
> fit in the existing IOMMU drivers model is difficult, but just reusing
> the Device Tree bindings seems feasible?

I started experimenting with that. As wrote in a separate email, I got a 
deferred probe timeout,

after inserting required nodes into guest device tree, which seems to be 
a consequence of the unavailability of IOMMU, I will continue to 
investigate this question.



-- 
Regards,

Oleksandr Tyshchenko