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

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Message-ID: <VI1PR0401MB263894265429130B91E7880C8DC80@VI1PR0401MB2638.eurprd04.prod.outlook.com>
Date:   Fri, 23 Sep 2016 16:09:29 +0000
From:   Stuart Yoder <stuart.yoder@....com>
To:     Arnd Bergmann <arnd@...db.de>, Robin Murphy <robin.murphy@....com>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        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



> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@...db.de]
> Sent: Friday, September 23, 2016 10:58 AM
> 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; kernel@...gutronix.de; Fabio Estevam Estevam
> <fabio.estevam@....com>; linux@...linux.org.uk; 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.

The reason we want aarch32 kernel support is for specific customers that
have requirements for legacy 32-bit kernel drivers that function on ARMv7
but for some reason are very problematic to port to a 64-bit kernel.  So,
the way to ease the 64-bit transition is allow them to run an aarch32 kernel
and their driver on an ARMv8 SoC.   I don't think we specifically care
whether the kernel starts at EL2 or EL1.

Stuart