Re: [PATCH v3 08/13] KVM: arm64: Add support for KVM_EXIT_HYPERCALL

[Marc Zyngier <maz@kernel.org>]

On Wed, 17 May 2023 19:38:14 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> Hi Salil,
> 
> On Wed, May 17, 2023 at 06:00:18PM +0000, Salil Mehta wrote:
> 
> [...]
> 
> > > > Should we expose the ESR, or at least ESR_EL2.IL as an additional
> > > > flag?
> > 
> > 
> > I think we would need "Immediate value" of the ESR_EL2 register in the
> > user-space/VMM to be able to construct the syndrome value. I cannot see
> > where it is being sent? 
> 
> The immediate value is not exposed to userspace, although by definition
> the immediate value must be zero. The SMCCC spec requires all compliant
> calls to use an immediate of zero (DEN0028E 2.9).
> 
> Is there a legitimate use case for hypercalls with a nonzero immediate?
> They would no longer be considered SMCCC calls at that point, so they
> wouldn't work with the new UAPI.

I agree. The use of non-zero immediate has long been deprecated. I
guess we should actually reject non-zero immediate for HVC just like
we do for SMC.

If there is an actual need for a non-zero immediate to be propagated
to userspace (want to emulate Xen's infamous 'HVC #0xEA1'?), then this
should be an extension to the current API.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.