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

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 09/03/2021 00:59, Stefano Stabellini wrote:
> On Sun, 7 Mar 2021, Julien Grall wrote:
>> On 06/03/2021 23:04, no-reply@patchew.org wrote:
>>> Hi,
>>>
>>> Patchew automatically ran gitlab-ci pipeline with this patch (series)
>>> applied, but the job failed. Maybe there's a bug in the patches?
>>>
>>> You can find the link to the pipeline near the end of the report below:
>>>
>>> Type: series
>>> Message-id: 20210306214148.27021-1-julien@xen.org
>>> Subject: [PATCH for-4.15] xen: Bump the minimum version of GCC supported to
>>> 4.9 (5.1 on arm64)
>>>
>>> === TEST SCRIPT BEGIN ===
>>> #!/bin/bash
>>> sleep 10
>>> patchew gitlab-pipeline-check -p xen-project/patchew/xen
>>> === TEST SCRIPT END ===
>>>
>>> warning: redirecting to https://gitlab.com/xen-project/patchew/xen.git/
>>> warning: redirecting to https://gitlab.com/xen-project/patchew/xen.git/
>>>   From https://gitlab.com/xen-project/patchew/xen
>>>    * [new tag]               patchew/20210306214148.27021-1-julien@xen.org ->
>>> patchew/20210306214148.27021-1-julien@xen.org
>>> Switched to a new branch 'test'
>>> f332859b88 xen: Bump the minimum version of GCC supported to 4.9 (5.1 on
>>> arm64)
>>>
>>> === OUTPUT BEGIN ===
>>> [2021-03-06 21:45:17] Looking up pipeline...
>>> [2021-03-06 21:45:18] Found pipeline 266507477:
>>>
>>> https://gitlab.com/xen-project/patchew/xen/-/pipelines/266507477
>>>
>>> [2021-03-06 21:45:18] Waiting for pipeline to finish...
>>> [2021-03-06 22:00:21] Still waiting...
>>> [2021-03-06 22:15:26] Still waiting...
>>> [2021-03-06 22:30:31] Still waiting...
>>> [2021-03-06 22:45:35] Still waiting...
>>> [2021-03-06 23:00:41] Still waiting...
>>> [2021-03-06 23:04:42] Pipeline failed
>>> [2021-03-06 23:04:42] Job 'qemu-smoke-x86-64-clang-pvh' in stage 'test' is
>>> skipped
>>> [2021-03-06 23:04:42] Job 'qemu-smoke-x86-64-gcc-pvh' in stage 'test' is
>>> skipped
>>> [2021-03-06 23:04:42] Job 'qemu-smoke-x86-64-clang' in stage 'test' is
>>> skipped
>>> [2021-03-06 23:04:42] Job 'qemu-smoke-x86-64-gcc' in stage 'test' is skipped
>>> [2021-03-06 23:04:42] Job 'qemu-smoke-arm64-gcc' in stage 'test' is skipped
>>> [2021-03-06 23:04:42] Job 'qemu-alpine-arm64-gcc' in stage 'test' is skipped
>>> [2021-03-06 23:04:42] Job 'build-each-commit-gcc' in stage 'test' is skipped
>>> [2021-03-06 23:04:42] Job 'alpine-3.12-clang' in stage 'build' is failed
>>
>> The following jobs are failing:
>>
>> alpine-3.12-clang [1]
>> debian-unstable-32-gcc-debug
>> debian-unstable-clang
>> debian-unstable-gcc-randconfig
>> ubuntu-bionic-clang
>>
>> Cloning into 'qemu-xen-dir-remote.tmp'...
>> error: inflate: data stream error (incorrect data check)
>> fatal: serious inflate inconsistency
>> fatal: index-pack failed
>>
>> This looks unrelated to my patch. Possibly a network error?
> 
> Yeah.
> 
> 
>> =========
>>
>> centos-7-2-gcc [2]
>> centos-7-2-gcc-debug
>> centos-7-gcc
>> centos-7-gcc-debug
>>
>> Centos 7 is using GCC 4.8.5. This patch is bumping the minimum version to 4.9.
>> So the failure is expected.
>>
>> We may have to disable the jobs if my patch goes ahead. May I ask why we chose
>> to build on Centos 7?
> 
> CentOS 7 is still the latest and it is maintained until 2024:
> 
> https://wiki.centos.org/About/Product
> 
> I don't have a lot of direct experience with x86 users any longer, but
> this seems to be problematic?

The last full updates was on the Q3 2020, so it looks unlikely that 
Centos 7 will upgrade to a new Xen.

So this is only going to be a problem for anyone that decides to build 
there own Xen.

I can't tell whether someone may want to build upstream Xen on Centos 7. 
What I can say is the Arm64 bug in GCC 4.9 will lead to corruption, the 
bug in GCC < 4.8 looks somewhat less problematic.

So, I am sure any user would appreciate if we tell (e.g. throw an error) 
them when there is a risk that GCC X.Y may miscompile Xen.

In the ideal world (e.g if we had unlimited time), we should workaround 
it in Xen. But this doesn't look easy/possible here.

Therefore, for now, I think dropping support for GCC < 4.9 (< 5.1 for 
Arm) is the best approach. The support can be revived when someone is 
willing to spend time on it.

> 
> 
>> ==========
>>
>> ubuntu-trusty-gcc [3]
>> ubuntu-trusty-gcc-debug
>>
>> Ubuntu trusty is using GCC 4.8.4. This patch is bumping the minimum version to
>> 4.9. So the failure is expected. We may have to disable the jobs if my patch
>> goes ahead. May I ask why we chose to build on Ubuntu trusty?
> 
> Ubuntu Trusty is in "Extended Security Maintenance", so technically
> still supported also:
> 
> https://ubuntu.com/about/release-cycle

See above.

Cheers,

-- 
Julien Grall