[PATCH 1/8] KVM: arm64: Don't corrupt tpidr_el2 on failed HVC call

[Quentin Perret]

On Monday 26 Oct 2020 at 09:51:09 (+0000), Marc Zyngier wrote:
> The hyp-init code starts by stashing a register in TPIDR_EL2
> in in order to free a register. This happens no matter if the
> HVC call is legal or not.
> 
> Although nothing wrong seems to come out of it, it feels odd
> to alter the EL2 state for something that eventually returns
> an error.
> 
> Instead, use the fact that we know exactly which bits of the
> __kvm_hyp_init call are non-zero to perform the check with
> a series of EOR/ROR instructions, combined with a build-time
> check that the value is the one we expect.

Alternatively, could we make __kvm_hyp_init non-SMCCC compliant? While I
understand how it makes sense to be compliant for 'proper' HVCs, this
one really is an odd one that only makes sense on a very transient state.
That would let us define our convention, and we can just say x0-x18 can
be clobbered like any function call, which eradicates the issue Andrew
tried to avoid with this tpidr_el2 trick.

Thoughts?

Thanks,
Quentin