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Subject: Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops
From:   Oleksandr <olekstysh@gmail.com>
To:     Stefano Stabellini <sstabellini@kernel.org>,
        xen-devel <xen-devel@lists.xenproject.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@lists.linux-foundation.org>,
        DTML <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Jason Wang <jasowang@redhat.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzk+dt@kernel.org>,
        Julien Grall <julien@xen.org>, Juergen Gross <jgross@suse.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>
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Date:   Mon, 23 May 2022 20:30:20 +0300
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On 19.05.22 09:03, Oleksandr wrote:

Hello Stefano, all


>
> On 19.05.22 04:06, Stefano Stabellini wrote:
>
>
> Hello Stefano, all
>
>> 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.


I have experimented with that. Yes, just reusing the Device Tree 
bindings is technically feasible (and we are able to do this by only 
touching grant-dma-ops.c), although deferred probe timeout still stands 
(as there is no IOMMU driver being present actually).

[    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, 
ignoring dependency
[    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks 
(2.10 GB/1.95 GiB)


Below the working diff (on top of current series):

diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index da9c7ff..6586152 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {

  bool xen_is_grant_dma_device(struct device *dev)
  {
+       struct device_node *iommu_np;
+       bool has_iommu;
+
         /* XXX Handle only DT devices for now */
         if (!dev->of_node)
                 return false;

-       return of_property_read_bool(dev->of_node, "xen,backend-domid");
+       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
+       has_iommu = iommu_np && of_device_is_compatible(iommu_np, 
"xen,grant-dma");
+       of_node_put(iommu_np);
+
+       return has_iommu;
  }

  void xen_grant_setup_dma_ops(struct device *dev)
  {
         struct xen_grant_dma_data *data;
-       uint32_t domid;
+       struct of_phandle_args iommu_spec;

         data = find_xen_grant_dma_data(dev);
         if (data) {
@@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
         if (!dev->of_node)
                 goto err;

-       if (of_property_read_u32(dev->of_node, "xen,backend-domid", 
&domid)) {
-               dev_err(dev, "xen,backend-domid property is not present\n");
+       if (of_parse_phandle_with_args(dev->of_node, "iommus", 
"#iommu-cells",
+                       0, &iommu_spec)) {
+               dev_err(dev, "Cannot parse iommus property\n");
+               goto err;
+       }
+
+       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
+                       iommu_spec.args_count != 1) {
+               dev_err(dev, "Incompatible IOMMU node\n");
+               of_node_put(iommu_spec.np);
                 goto err;
         }

+       of_node_put(iommu_spec.np);
+
         data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
         if (!data)
                 goto err;

-       data->backend_domid = domid;
+       /*
+        * The endpoint ID here means the ID of the domain where the 
corresponding
+        * backend is running
+        */
+       data->backend_domid = iommu_spec.args[0];

         if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, 
data,
                         GFP_KERNEL))) {
(END)


Below, the nodes generated by Xen toolstack:

         xen_grant_dma {
                 compatible = "xen,grant-dma";
                 #iommu-cells = <0x01>;
                 phandle = <0xfde9>;
         };

         virtio@2000000 {
                 compatible = "virtio,mmio";
                 reg = <0x00 0x2000000 0x00 0x200>;
                 interrupts = <0x00 0x01 0xf01>;
                 interrupt-parent = <0xfde8>;
                 dma-coherent;
                 iommus = <0xfde9 0x01>;
         };


I am wondering, would be the proper solution to eliminate deferred probe 
timeout issue in our particular case (without introducing an extra IOMMU 
driver)?


>
>
>
>
-- 
Regards,

Oleksandr Tyshchenko