Re: [PATCH for-4.15] xen: Bump the minimum version of GCC supported to 4.9 (5.1 on arm64)

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2021 21:22, Julien Grall wrote:
> On 08/03/2021 11:20, Jan Beulich wrote:
>> On 08.03.2021 11:51, Julien Grall wrote:
>>> On 08/03/2021 08:09, Jan Beulich wrote:
>>>> Additionally - partly related to your own reply regarding the CI
>>>> failures - imo there needs to be an analysis of what older distros
>>>> will no longer build (at all or by default).
>>>
>>> Per the CI, this would be Ubuntu Trusty (and older), Centos 7 (and older)
>>>
>>> Do you have any other in mind?
>>
>> Our SLE12 (latest service pack is SP5 and still has a while to go
>> to at least reach LTSS state) comes with gcc 4.8 as the default
>> compiler.
> 
> Thanks! That's good to know. Is it the old GCC Suse supports?

Not sure I understand the question: The default compiler of this or
any distro is of course (expected to be) supported by the vendor
for the lifetime of the OS.

>>>>> --- a/xen/include/xen/compiler.h
>>>>> +++ b/xen/include/xen/compiler.h
>>>>> @@ -5,6 +5,19 @@
>>>>>    #error Sorry, your compiler is too old/not recognized.
>>>>>    #endif
>>>>>    
>>>>> +#if CONFIG_CC_IS_GCC
>>>>> +# if CONFIG_GCC_VERSION < 40900
>>>>> +/* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 */
>>>>
>>>> As per the bug report, the issue was determined to not be present
>>>> in e.g. 4.3. Hence while such a bug may influence our choice of
>>>> minimum version, I don't think it can reasonably be named here as
>>>> the apparent only reason for the choice. Personally I don't think
>>>> any justification should be put here.
>>>
>>> Ok.
>>>
>>>>
>>>>> +#  error Sorry, your version of GCC is too old - please use 4.9 or newer.
>>>>> +# elif defined(CONFIG_ARM_64) && CONFIG_GCC_VERSION < 50100
>>>>> +/*
>>>>> + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63293
>>>>> + * https://lore.kernel.org/r/20210107111841.GN1551@xxxxxxxxxxxxxxxxxxxxx
>>>>> + */
>>>>> +#  error Sorry, your version of GCC is too old - please use 5.1 or newer.
>>>>
>>>>   From the bug entry the fix looks to have been backported to 4.9,
>>>> or at least some (important?) branches thereof.
>>>
>>> It is not clear what's you are trying to point out. Mind clarifying?
>>
>> Some 4.9 compilers (perhaps widely used ones) may not have the bad
>> issue, which puts under question their ruling out when the main
>> reason for doing so is that bug.
> 
> Well... We could surely try to hunt which GCC 4.9 has been fixed. But I 
> am not convinced this is useful, we would need to have an allowlist of 
> GCC compiler.

Or probe the compiler for the bug in question.

Jan