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

On Tue, May 25, 2021 at 06:13:58PM +0100, Marc Zyngier wrote:

On Tue, 25 May 2021 16:14:32 +0100,
Will Deacon [off-list ref] wrote:

quoted

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

[...]

quoted

+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, I'll add "and will remain non-runnable until re-initialised by a
subsequent KVM_ARM_VCPU_INIT operation".

Can the VMM tell that it needs to do that? I wonder if we should be
setting 'hardware_entry_failure_reason' to distinguish this case.

Will

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