Re: [PATCH v7 22/22] Documentation: arm64: describe asymmetric 32-bit support

Marc Zyngier
2021-05-25T17:17:02Z

On Tue, 25 May 2021 16:14:32 +0100,
Will Deacon <will@kernel.org> wrote:
>
> Document support for running 32-bit tasks on asymmetric 32-bit systems
> and its impact on the user ABI when enabled.
>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> .../admin-guide/kernel-parameters.txt | 3 +
> Documentation/arm64/asymmetric-32bit.rst | 154 ++++++++++++++++++
> Documentation/arm64/index.rst | 1 +
> 3 files changed, 158 insertions(+)
> create mode 100644 Documentation/arm64/asymmetric-32bit.rst
>

[...]

> +KVM
> +---
> +
> +Although KVM will not advertise 32-bit EL0 support to any vCPUs on an
> +asymmetric system, a broken guest at EL1 could still attempt to execute
> +32-bit code at EL0. In this case, an exit from a vCPU thread in 32-bit
> +mode will return to host userspace with an ``exit_reason`` of
> +``KVM_EXIT_FAIL_ENTRY``.

Nit: there is a bit more to it. The vcpu will be left in a permanent
non-runnable state until KVM_ARM_VCPU_INIT is issued to reset the vcpu
into a saner state.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.