Re: [PATCH v2.2 8/7] x86/IOMMU: Use altcall, and __initconst_cf_clobber

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 25/02/2022 08:24, Jan Beulich wrote:
> On 22.02.2022 12:47, Andrew Cooper wrote:
>> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
>> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
>> @@ -628,7 +628,7 @@ static void cf_check amd_dump_page_tables(struct domain 
>> *d)
>>                                hd->arch.amd.paging_mode, 0, 0);
>>  }
>>  
>> -static const struct iommu_ops __initconstrel _iommu_ops = {
>> +static const struct iommu_ops __initconst_cf_clobber _iommu_ops = {
> Following my initcall related remark on x86'es time.c I'm afraid I don't
> see how this and ...
>
>> @@ -2794,7 +2793,7 @@ static int __init cf_check 
>> intel_iommu_quarantine_init(struct domain *d)
>>      return rc;
>>  }
>>  
>> -static struct iommu_ops __initdata vtd_ops = {
>> +static const struct iommu_ops __initconst_cf_clobber vtd_ops = {
> ... this actually works. But I guess I must be overlooking something, as
> I'm sure that you did test the change.
>
> Both ops structures reference a function, through .adjust_irq_affinities,
> which isn't __init but which is used (besides here) for an initcall. With
> the ENDBR removed by the time initcalls are run, these should cause #CP.

This doesn't explode because the indirect calls are resolved to direct
calls before the ENDBR's are clobbered to NOP4.

But I really do need to write a proper sphinx doc explaining how this
works and the safety considerations.

~Andrew