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

[Julien Grall <julien@xxxxxxx>]

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@xxxxxxxxxxx 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@xxxxxxx
> > > 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@xxxxxxx ->
> > > patchew/20210306214148.27021-1-julien@xxxxxxx
> > > 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