Re: [PATCH v4 14/24] xen/domctl: wrap pci-subset iommu-related domctl op with CONFIG_MGMT_HYPERCALLS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.02.2026 09:23, Penny, Zheng wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: Wednesday, February 4, 2026 4:09 PM
>>
>> On 04.02.2026 08:50, Penny, Zheng wrote:
>>>> -----Original Message-----
>>>> From: Penny, Zheng <penny.zheng@xxxxxxx>
>>>> Sent: Friday, November 21, 2025 6:58 PM [...]
>>>> 772,14 +774,16 @@ static const struct iommu_ops
>>>> __initconst_cf_clobber  _iommu_ops = {
>>>>      .quarantine_init = amd_iommu_quarantine_init,
>>>>      .add_device = amd_iommu_add_device,
>>>>      .remove_device = amd_iommu_remove_device,
>>>> -    .assign_device  = amd_iommu_assign_device,
>>>>      .teardown = amd_iommu_domain_destroy,
>>>>      .clear_root_pgtable = amd_iommu_clear_root_pgtable,
>>>>      .map_page = amd_iommu_map_page,
>>>>      .unmap_page = amd_iommu_unmap_page,
>>>>      .iotlb_flush = amd_iommu_flush_iotlb_pages,
>>>> +#ifdef CONFIG_MGMT_HYPERCALLS
>>>> +    .assign_device  = amd_iommu_assign_device,
>>>>      .reassign_device = reassign_device,
>>>>      .get_device_group_id = amd_iommu_group_id,
>>>> +#endif
>>>
>>> FWIS, Alejandro has come up a more clever way to DCE these kinds of op,
>> staying conditionally as callback. Here, I just took this commit as example 
>> to show
>> the methodology:
>>> ```
>>>          .assign_device  = IS_ENABLED(CONFIG_MGMT_HYPERCALLS)
>>>                                         ? amd_iommu_assign_device
>>>                                         : NULL, ``` The compiler has
>>> enough visibility to know that static(amd_iommu_assign_device()) is used, 
>>> and is
>> droppable when MGMT_HYPERCALLS=n. So there is no need to do ifdef-wrapping
>> around these statics now. Later when jason's "--gc-section" patch serie in,  
>> --gc-
>> section will help linker identify them unused when MGMT_HYPERCALLS=n, then
>> remove them automatically.
>>
>> I fear I don't see why --gc-sections would make a difference when, for static
>> functions, the compiler already is in the position of removing the functions.
> 
> I may misunderstand the DCE process. With this change, we don't need to put 
> #ifdef CONFIG_MGMT_HYPERCALLS around the statics (e.g. 
> amd_iommu_assign_device()). So preprocessor will not help us remove them.

But the compiler's optimization passes can, when they observe that a static
function, while referenced by source code, is referenced only in branches of
conditionals which are compile-time false. Without link-time optimization
that's really about all DCE can do.

Jan