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

From: Marc Zyngier <maz@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>,
	kvm@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	linux-arm-kernel@lists.infradead.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [RFC PATCH 0/5] Removing support for 32bit KVM/arm host
Date: Thu, 20 Feb 2020 14:01:33 +0000	[thread overview]
Message-ID: <3f7f3b6c8b758b6d2134364616c6bc1e@kernel.org> (raw)
In-Reply-To: <b3faa8be-29ef-e637-bda6-ff76864ff388@arm.com>

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.

         M.
-- 
Jazz is not dead. It just smells funny...
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