[PATCH v4 02/20] arm64: fpsimd: Fix type mismatch in sme_{save,load}_state()

[Mark Rutland]

The sme_save_state() and sme_load_state() functions take a 32-bit int
argument that describes whether to save/restore ZT0. Their assembly
implementations consume the entire 64-bit register containing this
32-bit value, and will attempt to save/restore ZT0 if any bit of
that 64-bit register is non-zero.

Per the AAPCS64 parameter passing rules, the callee is responsible for
any necessary widening, and the upper 32-bits are permitted to contain
arbitrary values. If the upper 32 bits are non-zero, this could result
in an unexpected attempt to save/restore ZT0, and consequently could
lead to unexpected traps/undefs/faults.

In practice compilers are very unlikely to generate code where the upper
32-bits would be non-zero, but they are permitted to do so.

Fix this by only consuming the low 32 bits of the register, and update
comments accordingly.

Fixes: 95fcec713259 ("arm64/sme: Implement context switching for ZT0")
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Fuad Tabba <tabba at google.com>
Cc: James Morse <james.morse at arm.com>
Cc: Marc Zyngier <maz at kernel.org>
Cc: Mark Brown <broonie at kernel.org>
Cc: Oliver Upton <oupton at kernel.org>
Cc: Vladimir Murzin <vladimir.murzin at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: stable at vger.kernel.org
---
 arch/arm64/kernel/entry-fpsimd.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry-fpsimd.S b/arch/arm64/kernel/entry-fpsimd.S
index cb08d879cb4de..28af4bcd97a50 100644
--- a/arch/arm64/kernel/entry-fpsimd.S
+++ b/arch/arm64/kernel/entry-fpsimd.S
@@ -103,13 +103,13 @@ SYM_FUNC_END(sme_set_vq)
  * Save the ZA and ZT state
  *
  * x0 - pointer to buffer for state
- * x1 - number of ZT registers to save
+ * w1 - number of ZT registers to save
  */
 SYM_FUNC_START(sme_save_state)
 	_sme_rdsvl	2, 1		// x2 = VL/8
 	sme_save_za 0, x2, 12		// Leaves x0 pointing to the end of ZA
 
-	cbz	x1, 1f
+	cbz	w1, 1f
 	_str_zt 0
 1:
 	ret
@@ -119,13 +119,13 @@ SYM_FUNC_END(sme_save_state)
  * Load the ZA and ZT state
  *
  * x0 - pointer to buffer for state
- * x1 - number of ZT registers to save
+ * w1 - number of ZT registers to save
  */
 SYM_FUNC_START(sme_load_state)
 	_sme_rdsvl	2, 1		// x2 = VL/8
 	sme_load_za 0, x2, 12		// Leaves x0 pointing to the end of ZA
 
-	cbz	x1, 1f
+	cbz	w1, 1f
 	_ldr_zt 0
 1:
 	ret
-- 
2.30.2