Subject: Re: [RFC PATCH 0/5] Removing support for 32bit KVM/arm host

From: Marc Zyngier <[email-masked]>
Date: Wed, 19 Feb 2020 08:31:13 +0000

On Tue, 18 Feb 2020 22:37:12 +0100
Daniel Golle <[email-masked]> wrote:

Daniel,

Please keep people on cc, it helps with the discussion.

> > Message-ID: <[email-masked]>
> > 
> > KVM/arm was merged just over 7 years ago, and has lived a very quiet
> > life so far. It mostly works if you're prepared to deal with its
> > limitations, it has been a good prototype for the arm64 version,
> > but it suffers a few problems:
> > 
> > - It is incomplete (no debug support, no PMU)
> > - It hasn't followed any of the architectural evolutions
> > - It has zero users (I don't count myself here)  
> 
> Not true. At least I'm using KVM on CortexA7 (sun7i aka. Allwinner A20)
> and it has been quite useful until now (running low memory footprint
> OpenWrt-based guests on yocto/OpenEmbedded host)

OK, so we have a user!

> > - It is more and more getting in the way of new arm64 developments  
> 
> Can you elaborate more on how it is getting in the way? Just in terms
> of effort to maintain the necessary abstractions or does something
> really block ARM64 KVM support?

Useless abstractions are indeed one of the problems. Essentially,
KVM/arm has become a pile of empty stubs that are added to make the
thing compile. This doesn't bode well for the future.

The other aspect is that new features appearing on arm64 (nested virt,
enclaves, and potentially some more) are tearing the code-base apart,
and doing so while keeping 32bit alive and kicking would be a
challenge. I'm not saying it is impossible, just that it is hard, and
for very little gain, specially given that *nobody* contributes to the
port.

> > So here it is: unless someone screams and shows that they rely on
> > KVM/arm to be maintained upsteam, I'll remove 32bit host support
> > form the tree. One of the reasons that makes me confident nobody is
> > using it is that I never receive *any* bug report. Yes, it is perfect.
> > But if you depend on KVM/arm being available in mainline, please shout.  
> 
> I cetainly don't depend on it, meaning I'd have to replace hardware
> worth less than $100 in the near future, that's not too bad.
> And yes, it has just been working perfectly ;)

I'm really glad it works well. Note that it isn't like we are taking it
away from you. 5.6 will still have it, and I will still maintain it as
part of the stable tree. It is just that I've come to the sad
conclusion that as a community, we need to move on.

	M.

PS: and if you find a decent, reliable machine to replace your A20,
please let me know. All the arm64 SBCs I've seen are utter crap
compared to my Cubietruck...
-- 
Jazz is not dead. It just smells funny...

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