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

From: Robin Murphy <robin.murphy@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Russell King <linux@arm.linux.org.uk>,
	kvm@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Quentin Perret <qperret@google.com>,
	Christoffer Dall <Christoffer.Dall@arm.com>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 0/5] Removing support for 32bit KVM/arm host
Date: Thu, 20 Feb 2020 14:38:40 +0000	[thread overview]
Message-ID: <5a984189-78bb-2707-3714-13edcee9e8f5@arm.com> (raw)
In-Reply-To: <3f7f3b6c8b758b6d2134364616c6bc1e@kernel.org>

On 20/02/2020 2:01 pm, Marc Zyngier wrote:
> On 2020-02-20 13:32, Robin Murphy wrote:
>> On 20/02/2020 1:15 pm, Marc Zyngier wrote:
>>> Hi Marek,
>>>
>>> On 2020-02-20 12:44, Marek Szyprowski wrote:
>>>> Hi Marc,
>>>>
>>>> On 10.02.2020 15:13, 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)
>>>>> - It is more and more getting in the way of new arm64 developments
>>>>
>>>> That is a bit sad information. Mainline Exynos finally got everything
>>>> that was needed to run it on the quite popular Samsung Exynos5422-based
>>>> Odroid XU4/HC1/MC1 boards. According to the Odroid related forums it is
>>>> being used. We also use it internally at Samsung.
>>>
>>> Something like "too little, too late" springs to mind, but let's be
>>> constructive. Is anyone using it in a production environment, where
>>> they rely on the latest mainline kernel having KVM support?
>>>
>>> The current proposal is to still have KVM support in 5.6, as well as
>>> ongoing support for stable kernels. If that's not enough, can you please
>>> explain your precise use case?
>>
>> Presumably there's no *technical* reason why the stable subset of v7
>> support couldn't be stripped down and brought back private to arch/arm
>> if somebody really wants and is willing to step up and look after it?
> 
> There is no technical reason at all, just a maintenance effort.
> 
> The main killer is the whole MMU code, which I'm butchering with NV,
> and that I suspect Will will also turn upside down with his stuff.
> Not to mention the hypercall interface that will need a complete overhaul.
> 
> If we wanted to decouple the two, we'd need to make the MMU code, the
> hypercalls, arm.c and a number of other bits private to 32bit.

Right, the prospective kvm-arm maintainer's gameplan would essentially 
be an equivalent "move virt/kvm/arm to arch/arm/kvm" patch, but then 
ripping out all the Armv8 and GICv3 gubbins instead. Yes, there would 
then be lots of *similar* code to start with, but it would only diverge 
further as v8 architecture development continues independently.

Anyway, I just thought it seemed worth saying out loud, to reassure 
folks that a realistic middle-ground between "yay bye!" and "oh no the 
end of the world!" does exist, namely "someone else's problem" :)

Robin.