Re: [Xen-devel] [PATCH v2 2/4] x86/apic: force phys mode if interrupt remapping is disabled

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.12.2019 10:17, Roger Pau Monné wrote:
> On Tue, Dec 03, 2019 at 04:14:46PM +0100, Jan Beulich wrote:
>> On 29.11.2019 12:38, Roger Pau Monné  wrote:
>>> On Fri, Nov 29, 2019 at 12:28:49PM +0100, 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>
>>>
>>> This is missing a command line doc update and the logic below ignores
>>> a user-set x2apic_phys value.
>>
>> So what would the behavior be in your opinion when the user
>> has requested cluster mode? I can't see you do much other
>> than panic()-ing, perhaps it's better to override the request
>> (as you already do)?
> 
> I think panic'ing is fine, a user shouldn't be setting x2apic_phys
> unless they know what are doing, and then Xen changing it on the back
> of the user also doesn't seem fine.
> 
> A panic explaining that x2apic_phys=false is not supported and that
> the box can only be booted with x2apic phys mode should be fine IMO.

I can see this as a valid position to take. Personally, however, I
do think we should avoid failing to boot if we easily can. (Yes, we
should log the fact that we ignore a command line option in such a
case.)

Jan

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