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

[Will Deacon <will@kernel.org>]

On Tue, May 25, 2021 at 07:11:44PM +0100, Marc Zyngier wrote:
> On Tue, 25 May 2021 18:27:03 +0100,
> Will Deacon <will@kernel.org> wrote:
> > 
> > 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@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, I'll add "and will remain non-runnable until re-initialised by a
> > subsequent KVM_ARM_VCPU_INIT operation".
> 
> Looks good.

Cheers.

> > 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.
> 
> The VMM should be able to notice that something is amiss, as any
> subsequent KVM_RUN calls will result in -ENOEXEC being returned, and
> we document this as "the vcpu hasn't been initialized or the guest
> tried to execute instructions from device memory (arm64)".
> 
> However, there is another reason to get a "FAILED_ENTRY", and that if
> we get an Illegal Exception Return exception when entering the
> guest. That one should always be a KVM bug.
> 
> So yeah, maybe there is some ground to populate that structure with
> the appropriate nastygram (completely untested).
> 
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 24223adae150..cf50051a9412 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -402,6 +402,10 @@ struct kvm_vcpu_events {
>  #define KVM_PSCI_RET_INVAL		PSCI_RET_INVALID_PARAMS
>  #define KVM_PSCI_RET_DENIED		PSCI_RET_DENIED
>  
> +/* KVM_EXIT_FAIL_ENTRY reasons */
> +#define KVM_ARM64_FAILED_ENTRY_NO_AARCH32_ALLOWED	0xBADBAD32
> +#define KVM_ARM64_FAILED_ENTRY_INTERNAL_ERROR		0xE1215BAD

Heh, you and your magic numbers ;)

I'll leave it up to you as to whether you want to populate this -- I just
spotted it and thought it might help to indicate what went wrong. This is a
pretty daft situation to end up in so whether anybody would realistically
try to recover from it is another question entirely.

Will