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

[Will Deacon]

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