Re: [PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.03.2021 11:57, Roger Pau Monné wrote:
> On Tue, Mar 02, 2021 at 10:59:37AM +0100, Jan Beulich wrote:
>> On 01.03.2021 19:00, Roger Pau Monné wrote:
>>> On Mon, Mar 01, 2021 at 04:14:26PM +0100, Jan Beulich wrote:
>>>> On 01.03.2021 15:01, Roger Pau Monné wrote:
>>>>> On Mon, Mar 01, 2021 at 09:31:07AM +0100, Jan Beulich wrote:
>>>>>> In this case the compiler is recognizing that no valid array indexes
>>>>>> remain (in x2apic_cluster()'s access to per_cpu(cpu_2_logical_apicid,
>>>>>> ...)), but oddly enough isn't really consistent about the checking it
>>>>>> does (see the code comment).
>>>>>
>>>>> I assume this is because of the underlying per_cpu access to
>>>>> __per_cpu_offset using cpu as the index, in which case wouldn't it be
>>>>> better to place the BUG_ON there?
>>>>
>>>> Not sure, to be honest. It seemed more logical to me to place it
>>>> next to where the issue is.
>>>
>>> I'm not sure whether I would prefer to place it in per_cpu directly to
>>> avoid similar issues popping up in other parts of the code in the
>>> future?
>>
>> That's going to be a lot of BUG_ON(), and hence a lot of "ud2"
>> instances. Not something I'm keen on having. The more that it's
>> not the per_cpu() which triggers the warning, but separate
>> conditionals allowing the compiler to deduce value ranges of
>> variables.
> 
> Right. I don't see much other way around this then. Did you check with
> clang also?

No, I didn't. But if anything it would require further additions,
surely no less.

> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

Jan