Re: [PATCH v9 2/8] iommu/arm: Introduce iommu_add_dt_pci_sideband_ids API

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2025 11:47, Mykyta Poturai wrote:
> On 19.03.25 17:28, Jan Beulich wrote:
>> On 19.03.2025 16:21, Mykyta Poturai wrote:
>>> On 17.03.25 16:56, Jan Beulich wrote:
>>>> On 14.03.2025 14:34, Mykyta Poturai wrote:
>>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>>>>
>>>>> The main purpose of this patch is to add a way to register PCI device
>>>>> (which is behind the IOMMU) using the generic PCI-IOMMU DT bindings [1]
>>>>> before assigning that device to a domain.
>>>>>
>>>>> This behaves similarly to the existing iommu_add_dt_device API, except it
>>>>> handles PCI devices, and it is to be invoked from the add_device hook in 
>>>>> the
>>>>> SMMU driver.
>>>>>
>>>>> The function dt_map_id to translate an ID through a downstream mapping
>>>>> (which is also suitable for mapping Requester ID) was borrowed from Linux
>>>>> (v5.10-rc6) and updated according to the Xen code base.
>>>>>
>>>>> [1] 
>>>>> https://www.kernel.org/doc/Documentation/devicetree/bindings/pci/pci-iommu.txt
>>>>>
>>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>>>> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>>>>> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
>>>>> ---
>>>>> Regarding pci_for_each_dma_alias question: getting host bridge node
>>>>> directly seems like a simpler solution with the same result. AFAIU
>>>>> with pci_for_each_dma_alias in linux we would arrive to the host brige
>>>>> node anyway, but also try to call dt_map_id for each device along the
>>>>> way. I am not sure why exactly it is done this way in linux, as
>>>>> according to the pci-iommu.txt, iommu-map node can only be present in
>>>>> the PCI root.
>>>>>
>>>>> v8->v9:
>>>>> * replace DT_NO_IOMMU with 1
>>>>> * guard iommu_add_pci_sideband_ids with CONFIG_ARM
>>>>
>>>> I fear I'm confused: Isn't this contradicting ...
>>>>
>>>>> v7->v8:
>>>>> * ENOSYS->EOPNOTSUPP
>>>>> * move iommu_add_pci_sideband_ids to iommu.c to fix x86 build
>>>>
>>>> ... this earlier change? Really, with there being no caller, I can't see
>>>> why there could be any build issue here affecting only x86. Except for
>>>> Misra complaining about unreachable code being introduced, which I'm sure
>>>> I said before should be avoided.
>>>
>>> The original reason for moving this function was the conflicting ACPI
>>> and EFI headers, I described it in V8 comments here[1].
>>>
>>>>
>>>>> --- a/xen/drivers/passthrough/iommu.c
>>>>> +++ b/xen/drivers/passthrough/iommu.c
>>>>> @@ -20,6 +20,7 @@
>>>>>    #include <xen/param.h>
>>>>>    #include <xen/softirq.h>
>>>>>    #include <xen/keyhandler.h>
>>>>> +#include <xen/acpi.h>
>>>>>    #include <xsm/xsm.h>
>>>>>
>>>>>    #ifdef CONFIG_X86
>>>>> @@ -744,6 +745,20 @@ int __init 
>>>>> iommu_get_extra_reserved_device_memory(iommu_grdm_t *func,
>>>>>        return 0;
>>>>>    }
>>>>>
>>>>> +#ifdef CONFIG_ARM
>>>>
>>>> I realize we have CONFIG_X86 here as well (visible even in context of the
>>>> earlier hunk. Yet then the goal ought to be to reduce these anomalies, not
>>>> add new ones. Since I don't have a clear picture of what's wanted, I'm also
>>>> in trouble suggesting any alternative, I'm afraid.
>>>
>>> Here is a short summary:
>>>
>>> The main problem is that we need this function somewhere, but there is
>>> no good place for it. It is only called on ARM for now but it's not
>>> ARM-specific by nature and can be eventually used on other platforms as
>>> well. It can't be just dropped because of the effort to support the
>>> co-existence of DT and ACPI. It also can't be declared as a static
>>> function because it requires the inclusion of <xen/acpi.h> for
>>> acpi_disabled define, which leads to build errors[1]. And without ifdef
>>> guards it would be a MISRA violation.
>>
>> An abridged version of this ought to go in the patch description, I think.
>> This is special, so it needs calling out.
>>
>> As to the placement - would making an entirely new .c file possibly help?
>> (Then, instead of in the patch description, maybe the special aspect could
>> be put in a code comment at the top of the file.)
> 
> It seems to me creating a new file would be overkill for one small 
> function. I considered moving it to xen/drivers/passthrough/arm/iommu.c 
> to reduce ifdefs but I feared it would suggest that it is arch-specific 
> a bit too strongly. So maybe move it there after all if you think it 
> would be better?

Well - with "#ifdef CONFIG_ARM" around it's Arm-specific too, isn't it?
IOW - either have a proper (even if simple) abstraction, or perhaps indeed
put it there (if that's also okay with the Arm maintainers).

Jan