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

["Roger Pau Monné" <royger@FreeBSD.org>]

On Mon, Nov 08, 2021 at 08:40:59AM +0100, Jan Beulich wrote:
> 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=".

I haven't looked very deeply, but will the acpi helpers work correctly
in that case? As acpi_boot_table_init will be short-circuited if
 `acpi_disabled && !acpi_ht`.

Thanks, Roger.