Re: [PATCH v2 1/6][4.16?] x86/x2APIC: defer probe until after IOMMU ACPI table parsing

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.11.2021 16:38, Roger Pau Monné wrote:
> On Fri, Nov 05, 2021 at 01:32:18PM +0100, Jan Beulich wrote:
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -1699,6 +1699,13 @@ void __init noreturn __start_xen(unsigne
>>  
>>      dmi_scan_machine();
>>  
>> +    /*
>> +     * IOMMU-related ACPI table parsing has to happen before APIC probing, 
>> for
>> +     * check_x2apic_preenabled() to be able to observe respective findings, 
>> in
>> +     * particular iommu_intremap having got turned off.
>> +     */
>> +    acpi_iommu_init();
> 
> If we pull this out I think we should add a check for acpi_disabled
> and if set turn off iommu_intremap and iommu_enable?

Hmm, I should have added a note regarding this. If we want to exactly
retain prior behavior, acpi_ht would also need checking. Yet that has
gone wrong long ago: We parse way too many tables when acpi_disabled
&& acpi_ht, and hence while correct wrt to prior behavior I'd consider
it wrong to (re)add a "!acpi_ht" check.

As a result I'm of the opinion that checking acpi_disabled here also
isn't necessarily appropriate, and instead IOMMU disabling would
better be solely under the control of "iommu=".

Additionally iirc Andrew has been suggesting to drop all this "ACPI
disabled / HT-only" machinery (I'm somewhat hesitant with that, but
as a result I'm also not very eager to actually correct to accumulated
bad behavior). The change here simply would be a tiny first step in
that direction.

Jan