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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan