Re: [Xen-devel] [PATCH v2 2/2] AMD/IOMMU: without XT, x2APIC needs to be forced into physical mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.02.2020 13:31, Roger Pau Monné wrote:
> On Fri, Feb 28, 2020 at 01:12:03PM +0100, Jan Beulich wrote:
>> --- a/xen/arch/x86/genapic/x2apic.c
>> +++ b/xen/arch/x86/genapic/x2apic.c
>> @@ -236,12 +236,21 @@ const struct genapic *__init apic_x2apic
>>          x2apic_phys = !iommu_intremap ||
>>                        (acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
>>      }
>> -    else if ( !x2apic_phys && !iommu_intremap )
>> -    {
>> -        printk("WARNING: x2APIC cluster mode is not supported without 
>> interrupt remapping\n"
>> -               "x2APIC: forcing phys mode\n");
>> -        x2apic_phys = true;
>> -    }
>> +    else if ( !x2apic_phys )
>> +        switch ( iommu_intremap )
>> +        {
>> +        case iommu_intremap_off:
>> +        case iommu_intremap_restricted:
>> +            printk("WARNING: x2APIC cluster mode is not supported %s 
>> interrupt remapping\n"
>> +                   "x2APIC: forcing phys mode\n",
>> +                   iommu_intremap == iommu_intremap_off ? "without"
>> +                                                        : "with 
>> restricted");
>> +            x2apic_phys = true;
> 
> I think you also need to fixup the usage of iommu_intremap in __cpu_up
> so that CPUs with APIC IDs > 255 are not brought up when in
> iommu_intremap_restricted mode.

I've looked around some (finding indications that, as one would
suspect, broadcasting is also supported for IO-APIC based
interrupts, and then by implication also for MSI) and then also
thought about this some more. I think the corner case here
resolves like this: Whether 0xff means "broadcast" exclusively
depends on the local APIC. Hence in x2APIC mode, even without
XT, 0xff does not mean "broadcast", and hence the code in
__cpu_up() is fine as is. In this setup there simply is no way
to encode broadcast at the IO-APIC or MSI level; luckily we
also don't use this mode.

Jan

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