KVM: arm64: Expose SMC/HVC width to userspace

When returning to userspace to handle a SMCCC call, we consistently
set PC to point to the instruction immediately after the HVC/SMC.

However, should userspace need to know the exact address of the
trapping instruction, it needs to know about the *size* of that
instruction. For AArch64, this is pretty easy. For AArch32, this
is a bit more funky, as Thumb has 16bit encodings for both HVC
and SMC.

Expose this to userspace with a new flag that directly derives
from ESR_EL2.IL. Also update the documentation to reflect the PC
state at the point of exit.

Finally, this fixes a small buglet where the hypercall.{args,ret}
fields would not be cleared on exit, and could contain some
random junk.
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/86pm8iv8tj.wl-maz@kernel.org

Documentation/virt/kvm/api.rst

@@ -6244,6 +6244,14 @@ Definition of ``flags``:
    conduit to initiate the SMCCC call. If this bit is 0 then the guest
    used the HVC conduit for the SMCCC call.
 
+ - ``KVM_HYPERCALL_EXIT_16BIT``: Indicates that the guest used a 16bit
+   instruction to initiate the SMCCC call. If this bit is 0 then the
+   guest used a 32bit instruction. An AArch64 guest always has this
+   bit set to 0.
+
+At the point of exit, PC points to the instruction immediately following
+the trapping instruction.
+
 ::
 
 		/* KVM_EXIT_TPR_ACCESS */

arch/arm64/include/uapi/asm/kvm.h

@@ -491,7 +491,8 @@ struct kvm_smccc_filter {
 };
 
 /* arm64-specific KVM_EXIT_HYPERCALL flags */
-#define KVM_HYPERCALL_EXIT_SMC	(1U << 0)
+#define KVM_HYPERCALL_EXIT_SMC		(1U << 0)
+#define KVM_HYPERCALL_EXIT_16BIT	(1U << 1)
 
 #endif
 

arch/arm64/kvm/hypercalls.c

@@ -222,13 +222,19 @@ static void kvm_prepare_hypercall_exit(struct kvm_vcpu *vcpu, u32 func_id)
 {
 	u8 ec = ESR_ELx_EC(kvm_vcpu_get_esr(vcpu));
 	struct kvm_run *run = vcpu->run;
-
-	run->exit_reason = KVM_EXIT_HYPERCALL;
-	run->hypercall.nr = func_id;
-	run->hypercall.flags = 0;
+	u64 flags = 0;
 
 	if (ec == ESR_ELx_EC_SMC32 || ec == ESR_ELx_EC_SMC64)
-		run->hypercall.flags |= KVM_HYPERCALL_EXIT_SMC;
+		flags |= KVM_HYPERCALL_EXIT_SMC;
+
+	if (!kvm_vcpu_trap_il_is32bit(vcpu))
+		flags |= KVM_HYPERCALL_EXIT_16BIT;
+
+	run->exit_reason = KVM_EXIT_HYPERCALL;
+	run->hypercall = (typeof(run->hypercall)) {
+		.nr	= func_id,
+		.flags	= flags,
+	};
 }
 
 int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)