linux-kernel - Re: [PATCH 1/2] armv8: aarch32: Execute 32-bit Linux for LayerScape platforms

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Date:   Fri, 23 Sep 2016 17:58:11 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Stuart Yoder <stuart.yoder@....com>,
        Alison Wang <b18965@...escale.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Fabio Estevam Estevam <fabio.estevam@....com>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Scott Wood <scott.wood@....com>, Leo Li <leoyang.li@....com>,
        Jason Jin <jason.jin@....com>
Subject: Re: [PATCH 1/2] armv8: aarch32: Execute 32-bit Linux for LayerScape platforms

On Friday, September 23, 2016 4:13:30 PM CEST Robin Murphy wrote:
> On 23/09/16 15:44, Arnd Bergmann wrote:
> > On Friday, September 23, 2016 3:24:12 PM CEST Robin Murphy wrote:
> >> On 23/09/16 15:01, Stuart Yoder wrote:
> >> Otherwise you can
> >> always simply run your own shim at EL2 to drive an AArch32 EL1 (it'll
> >> need to trap and translate subsequent SMC calls for e.g. PSCI).
> >>
> >>> If there is such a requirement, it's something begging for standardization.
> >>> Doesn't make sense for multiple divergent approaches for switching from
> >>> aarch64/EL2 to aarch32/EL2.
> >>
> >> Perhaps - I did briefly look into how hard it would be to write a proper
> >> SMC service handler to do this (since ATF does have a framework for such
> >> things), but concluded it would be more than 10 minutes' work and just
> >> cheated instead. It's certainly something which could be raised with the
> >> firmware folks.
> > 
> > If we end up allowing all arm64 platforms to be enabled in arch/arm,
> > we could perhaps create a generic implementation that does both of
> > those things, i.e.
> > 
> > - Take the arm32 kernel Image or zImage file, wrap it inside of a binary
> >   that implements the arm64 boot protocol.
> > - When that starts up, try to use the new PSCI call to jump into
> >   the arm32 kernel
> > - If PSCI failed and we are running in EL2, implement the shim
> >   and start the arm32 kernel in EL1 mode
> 
> Really, though, the firmware call thing is an incredibly niche use-case.
> Beyond development, the only real benefit of starting an AArch32 kernel
> in Hyp is that you can run AArch32 KVM guests, which you can do equally
> well (if not better) under an AArch64 kernel.

This was my question earlier in the thread, apparently Alison has
another use case in mind, but I don't yet know what that is. If
that use case is important enough, we could do it this way.

The only use case I can think of at the moment is boot testing
on kernelci.org, which could be used to check whether all the drivers
work in 32-bit environments.

	Arnd