linux-kernel - [PATCH v4 2/5] KVM: arm64: Zero x4-x7 in ffa_set

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Message-Id: <20250516-virtio-msg-ffa-v4-2-580ee70e5081@google.com>
Date: Fri, 16 May 2025 12:14:01 +0000
From: Per Larsen via B4 Relay <devnull+perlarsen.google.com@...nel.org>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
 Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
 Zenghui Yu <yuzenghui@...wei.com>, 
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
 Sudeep Holla <sudeep.holla@....com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, sebastianene@...gle.com, qperret@...gle.com, 
 qwandor@...gle.com, arve@...roid.com, perl@...unant.com, 
 lpieralisi@...nel.org, kernel-team@...roid.com, tabba@...gle.com, 
 james.morse@....com, armellel@...gle.com, jean-philippe@...aro.org, 
 ahomescu@...gle.com, Per Larsen <perlarsen@...gle.com>
Subject: [PATCH v4 2/5] KVM: arm64: Zero x4-x7 in ffa_set_retval

From: Per Larsen <perlarsen@...gle.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@...gle.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