Re: [PATCH] KVM: arm64: Pass a 64bit function-id in the SMC handlers

Marc Zyngier
2026-04-01T18:28:43Z

On Wed, 01 Apr 2026 18:21:58 +0100,
Sebastian Ene <sebastianene@google.com> wrote:
>
> On Wed, Apr 01, 2026 at 03:55:11PM +0100, Marc Zyngier wrote:
> > On Wed, 01 Apr 2026 13:32:01 +0100,
> > Sebastian Ene <sebastianene@google.com> wrote:
> > >
> > > Make the SMC handlers accept a 64bit value for the function-id to keep
> > > it uniform with the rest of the code and prevent a u64 -> u32 -> u64
> > > conversion as it currently happens when we handle PSCI.
> >
> > That seems overly creative. The spec says (2.5, from ARM DEN 0028 1.6
> > G):
>
> I'm not plannig to be *overly creative*. Thanks for pointing out the ARM
> spec.
>
> >
> > "The Function Identifier is passed on W0 on every SMC and HVC
> > call. Its 32-bit integer value indicates which function is being
> > requested by the caller. It is always passed as the first argument to
> > every SMC or HVC call in R0 or W0."
> >
> > which indicates that it is *always* a 32bit value.
> >
> > So if you have a 64bit value somewhere, *that* should be fixed, not
> > propagated arbitrarily.
>
> If you have a non SMCCC call that happen to have the first 32-bits of
> the function-id matching either PSCI or FF-A you will end up handling
> them instead of forwarding it to Trustzone because func_id is declared as:
>
> DECLARE_REG(u64, func_id, host_ctxt, 0);

Again, the correct approach to prevent the propagation of something
that is known to be wrong. Something like this:

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 007fc993f2319..dae993a1d081b 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -694,6 +694,11 @@ static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
DECLARE_REG(u64, func_id, host_ctxt, 0);
bool handled;

+ if (upper_32_bits(func_id)) {
+ cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
+ kvm_skip_host_instr();
+ }
+
func_id &= ~ARM_SMCCC_CALL_HINTS;

handled = kvm_host_psci_handler(host_ctxt, func_id);

Because forwarding something that is blatantly illegal to the secure
side is not something we should accept at all (just like we now refuse
non-zero SMC immediate values).

Thanks,

M.

--
Jazz isn't dead. It just smells funny.