Re: [PATCH 1/3] KVM: arm64: Restrict FF-A host version renegotiation

[Marc Zyngier]

On Fri, 02 May 2025 04:52:39 +0100,
Per Larsen <perl@xxxxxxxxxxxx> wrote:
> 
> FF-A implementations with the same major version must interoperate with
> earlier minor versions per DEN0077A 1.2 REL0 13.2.1 but FF-A version 1.1
> broke the ABI on several structures and 1.2 relies on SMCCC 1.2 is not
> backwards compatible with SMCCC 1.2 (see DEN0028 1.6 G BET0 Appendix F).
> 
> If we return the negotiated hypervisor version when the host requests a
> lesser minor version, the host will rely on the FF-A interoperability
> rules. Since the hypervisor does not currently have the necessary
> compatibility paths (e.g. to handle breaking changes to the SMC calling
> convention), return NOT_SUPPORTED.
> 
> Signed-off-by: Per Larsen <perlarsen@xxxxxxxxxx>
> Signed-off-by: Per Larsen <perl@xxxxxxxxxxxx>
> ---
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index 3369dd0c4009..10e88207b78e 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -712,7 +712,24 @@ static void do_ffa_version(struct arm_smccc_res *res,
> 
>   hyp_spin_lock(&version_lock);
>   if (has_version_negotiated) {
> - res->a0 = hyp_ffa_version;
> + /*
> + * FF-A implementations with the same major version must
> + * interoperate with earlier minor versions per DEN0077A 1.2
> + * REL0 13.2.1 but FF-A version 1.1 broke the ABI on several
> + * structures and 1.2 relies on SMCCC 1.2 is not backwards
> + * compatible with SMCCC 1.2 (see DEN0028 1.6 G BET0 Appendix F).

I can't parse this sentence. Missing words?

> + *
> + * If we return the negotiated hypervisor version when the host
> + * requests a lesser minor version, the host will rely on the
> + * aforementioned FF-A interoperability rules. Since the
> + * hypervisor does not currently have the necessary compatibility
> + * paths (e.g. to paper over the above-mentioned calling
> + * convention changes), return NOT_SUPPORTED.
> + */
> + if (FFA_MINOR_VERSION(ffa_req_version) < FFA_MINOR_VERSION(hyp_ffa_version))
> + res->a0 = FFA_RET_NOT_SUPPORTED;
> + else
> + res->a0 = hyp_ffa_version;
>   goto unlock;
>   }
> 

Something has gone seriously wrong with your email, and the patches
are badly mangled and unusable. They are also sent as individual
patches and not as a thread, which is a sign that you didn't send them
using git. Please fix this for your next posting.

More to the meat of the patches: why should the hypervisor paper over
anything if the spec is broken? Why can't the host just as well decide
for itself what to do?

Thanks,

	M.

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