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

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Date:   Tue, 25 May 2021 18:27:03 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        kernel-team@...roid.com
Subject: 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 <will@...nel.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@...nel.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, 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