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

[Arnd Bergmann <arnd@arndb.de>]

On Sat, Feb 22, 2020 at 3:40 PM Takashi Yoshi <takashi@yoshi.email> wrote:
> On Monday, 10.02.2020, 14:13 +0000 Marc Zyngier wrote:
> > 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)
>
> I might not be an important user, but I have been for multiple years
> and still am a regular user of KVM/arm32 on different devices.
>
> I use KVM on my Tegra K1 Chromebook for app development and have
> multiple SBCs at home on which I run VMs on using KVM+libvirt.
>
> Sure, neither of these devices has many resources available, but they
> are working fine. I would love to keep them in service since I haven't
> found arm64-based replacements that don't require hours upon hours of
> tinkering to just get a basic OS installation running with a mainline
> kernel.
>
> As an example that they can still be of use in 2020 I'd like to point
> out that one of the SBCs is running my DNS resolver, LDAP server,
> RSS reader, IRC bouncer, and shared todo list just fine, each in their
> separate VM.

Thank you for providing an important data point to this question.

> > - It is more and more getting in the way of new arm64 developments
> >
> > 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.
>
> *scream*
>
> > One of the reasons that makes me confident nobody is
> > using it is that I never receive *any* bug report. Yes, it is
> > perfect.
>
> This assumption is deeply flawed. Most users (including me) are not
> subscribed to this mailing list and will never find this thread at all.
> I myself stumbled upon this discussion just by chance while I was
> browsing the web trying to find something completely unrelated.
>
> I've been using KVM on x86, ppc and arm for many years, yet I never
> felt the need to report a bug on the mailing list.
> (This is to be interpreted as a compliment to the great work the devs
> of KVM have done!)
>
> Just going by the number of bugs reported on a developers mailing list
> is not going to paint an accurate picture.
>
> I am convinced that I'm not the only one relying on KVM/arm32 in the
> mainline kernel and would ask you to please reconsider keeping arm32 in
> the mainline kernel for a few more years until adequate arm64
> replacements are available on the market and have gained proper support
> in the mainline kernel.

Can you provide some more information about how you use KVM on 32-bit
machines, to make it possible to better estimate how many others might
do the same, and how long you will need to upgrade to newer kernels for?

In particular:

- What is the smallest amount of physical RAM that you have to found to
   make a usable ARM/KVM host? Note that the 4GB configuration of the
   Tegra K1 (an rk3288) Chromebooks seems to be extremely rare in other
   devices, while most new 32-bit SBCs come with 1GB or less these days.

- How often do you update the host kernels on those 32-bit machines that
  you still use to newer releases? What is the oldest/newest you run at the
  moment?

- Are you able to move the host installation to a distribution with a long-term
  stable release cycle such as Debian, Ubuntu that gives you a ~five year
  support after a kernel release?

         Arnd
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