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

[Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>]

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.
>
> Jan

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.


[1]
https://patchew.org/Xen/cover.1739182214.git.mykyta._5Fpoturai@xxxxxxxx/67b9bd138c12c0df35e5c4b3137c30ad345097d5.1739182214.git.mykyta._5Fpoturai@xxxxxxxx/#e6ea52a3-c7ce-411f-8186-cf725aa608f3@xxxxxxxx
--
Mykyta