> -----Original Message----- > From: Arnd Bergmann [mailto:a...@arndb.de] > Sent: Friday, September 23, 2016 10:58 AM > To: Robin Murphy <robin.mur...@arm.com> > Cc: linux-arm-ker...@lists.infradead.org; Stuart Yoder > <stuart.yo...@nxp.com>; Alison Wang > <b18...@freescale.com>; shawn...@kernel.org; ker...@pengutronix.de; Fabio > Estevam Estevam > <fabio.este...@nxp.com>; li...@armlinux.org.uk; linux-kernel@vger.kernel.org; > Scott Wood > <scott.w...@nxp.com>; Leo Li <leoyang...@nxp.com>; Jason Jin > <jason....@nxp.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
