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

[Marc Zyngier]

On Tue, 25 May 2021 16:14:32 +0100,
Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  .../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.