[PATCH v4 2/5] KVM: arm64: Zero x4-x7 in ffa_set_retval

[Per Larsen <perlarsen@google.com>]

FF-A 1.0 references SMCCC 1.2 (reference [4] on page xi). The results of
an SMC32/HVC32 are returned in W1-W7 in SMCCC 1.2. However, W4-W7 must be
preserved unless they contain results according to the function definition.

ffa_set_retval is commonly used to report FFA_SUCCESS or FFA_ERROR back to
the caller. FFA_SUCCESS requires that x2-x7 must be zero (MBZ) unless a
register is explicitly used in the function response. FFA_ERROR requires
x3-x7 MBZ.

ffa_set_retval is also used to return results of:
- FFA_VERSION which requires x1-x7 MBZ
- FFA_MEM_FRAG_TX which returns FFA_MEM_FRAG_RX or FFA_ERROR.
  FFA_MEM_FRAG_RAX always requires that x5-x7 MBZ and x4 MBZ
  at any virtual FF-A instance (applies to kvm).

Messaging interfaces such as FFA_MSG_SEND_DIRECT_{REQ,REQ2}, will not use
ffa_set_retval.

Given the above, it is safe to zero x4-x7 in ffa_set_retval.

Signed-off-by: Per Larsen <perlarsen@google.com>
---
 arch/arm64/kvm/hyp/nvhe/ffa.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
index 2c199d40811efb5bfae199c4a67d8ae3d9307357..b3d016bee404ce3f8c72cc57befb4ef4e6c1657f 100644
--- a/arch/arm64/kvm/hyp/nvhe/ffa.c
+++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
@@ -101,6 +101,10 @@ static void ffa_set_retval(struct kvm_cpu_context *ctxt,
 	cpu_reg(ctxt, 1) = res->a1;
 	cpu_reg(ctxt, 2) = res->a2;
 	cpu_reg(ctxt, 3) = res->a3;
+	cpu_reg(ctxt, 4) = 0;
+	cpu_reg(ctxt, 5) = 0;
+	cpu_reg(ctxt, 6) = 0;
+	cpu_reg(ctxt, 7) = 0;
 }
 
 static bool is_ffa_call(u64 func_id)

-- 
2.49.0.1101.gccaa498523-goog