Re: [Xen-devel] [PATCH v3 2/4] x86/apic: force phys mode if interrupt remapping is disabled

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.12.2019 11:25, Roger Pau Monné wrote:
> On Thu, Dec 05, 2019 at 10:32:34AM +0100, Jan Beulich wrote:
>> On 04.12.2019 17:20, Roger Pau Monne wrote:
>>> Cluster mode can only be used with interrupt remapping support, since
>>> the top 16bits of the APIC ID are filled with the cluster ID, and
>>> hence on systems where the physical ID is still smaller than 255 the
>>> cluster ID is not. Force x2APIC to use physical mode if there's no
>>> interrupt remapping support.
>>>
>>> Note that this requires a further patch in order to enable x2APIC
>>> without interrupt remapping support.
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> albeit ...
>>
>>> --- a/xen/arch/x86/genapic/x2apic.c
>>> +++ b/xen/arch/x86/genapic/x2apic.c
>>> @@ -226,7 +226,23 @@ boolean_param("x2apic_phys", x2apic_phys);
>>>  const struct genapic *__init apic_x2apic_probe(void)
>>>  {
>>>      if ( x2apic_phys < 0 )
>>> -        x2apic_phys = !!(acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
>>> +    {
>>> +        if ( !iommu_intremap )
>>> +            /*
>>> +             * Force physical mode if there's no interrupt remapping 
>>> support:
>>> +             * the ID in clustered mode requires a 32 bit destination 
>>> field due
>>> +             * to the usage of the high 16 bits to store the cluster ID.
>>> +             */
>>> +            x2apic_phys = true;
>>> +        else
>>> +            x2apic_phys = !!(acpi_gbl_FADT.flags & 
>>> ACPI_FADT_APIC_PHYSICAL);
>>
>> ... I wonder why you didn't make this
>>
>>         x2apic_phys = !iommu_intremap || (acpi_gbl_FADT.flags & 
>> ACPI_FADT_APIC_PHYSICAL);
>>
>> (not the least because of allowing to drop the somewhat ugly !!).
> 
> Feel free to do it at commit (and reindent the comment), or else I can
> resend a new version if that's too intrusive.

Doing these adjustments at commit time ought to be fine. It's
just that I'd prefer to wait with committing this series until
4.13 is fully finished.

Jan

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