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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 20 Mar 2025, Jan Beulich wrote:
> 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).

I am OK with that