linux-kernel - Re: [GIT PULL] KVM, AMD PSP and ARM CoreSight changes for 5.13 merge window

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Date:   Mon, 3 May 2021 18:09:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [GIT PULL] KVM, AMD PSP and ARM CoreSight changes for 5.13 merge
 window

On 03/05/21 17:25, Mathieu Poirier wrote:
>> Mathieu, can you confirm that your coresight branch will*not*  be sent by
>> the ARM maintainers as well?
> Confirmed.  Marc's tree is the only place where the ETE-TRBE functionality has
> been added.  It was specifically done that way to avoid having the same code in
> multiple branches and prevent merge conflicts.

Thanks for confirming!

Generally, what we do for x86 is exactly the opposite: the basic 
functionality is committed to the x86 tree, and then merged in _also_ by 
myself.  For example, this pull request includes a topic branch provided 
by the cgroup maintainer and one provided by the x86 maintainers, but in 
both cases they _also_ sent exactly the same commits to Linus.

It works well because git is pretty good at avoiding conflicts when the 
same branch is present in multiple branches.  Instead, cherry-picking 
introduces lots of merge conflicts.

There are other advantages in doing that.  For example, in this case I 
didn't (and don't) quite know what ETE and TRBE are, beyond what a quick 
Internet search tells me.  Sending this functionality to an ARM 
maintainer that is more acquainted with the feature would ensure that 
the new functionality is documented properly in the tags and therefore 
in the commit messages.

This is what Linux was mentioning when he said "Pull requests need to 
have explanations of what they pull - not just because it needs to go 
into the merge message, but because the maintainer needs to keep track 
of what's happening".  In this case, the maintainer was me; based on my 
own workflow and due to the lack of commit message I assumed that the 
branch was also going to go through the ARM tree (and doubted my 
assumption only when sending the pull request to Linus, i.e. way too late).

I am also guilty as charged of the "Merge branch 'kvm-sev-cgroup' into 
HEAD" commit message, where I should have pointed out that Tejun had a 
later branchpoint from 5.12-rc than I did, resulting in conflicts.

So Marc, let's heed Linus's advice and document all topic branches that 
we merge into the KVM/ARM and KVM/x86 trees, including whether they also 
go into other trees---which they should do almost all the time.

Thanks,

Paolo

> Let me know if you need more information.