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

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