kselftest/arm64: Implement irritators for ZA and ZT

Currently we don't use the irritator signal in our floating point stress
tests so when we added ZA and ZT stress tests we didn't actually bother
implementing any actual action in the handlers, we just counted the signal
deliveries. In preparation for using the irritators let's implement them,
just trivially SMSTOP and SMSTART to reset all bits in the register to 0.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241107-arm64-fp-stress-irritator-v2-4-c4b9622e36ee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

tools/testing/selftests/arm64/fp/za-test.S

@@ -148,20 +148,16 @@ function check_za
 	b	memcmp
 endfunction
 
-// Any SME register modified here can cause corruption in the main
-// thread -- but *only* the locations modified here.
+// Modify the live SME register state, signal return will undo our changes
 function irritator_handler
 	// Increment the irritation signal count (x23):
 	ldr	x0, [x2, #ucontext_regs + 8 * 23]
 	add	x0, x0, #1
 	str	x0, [x2, #ucontext_regs + 8 * 23]
 
-	// Corrupt some random ZA data
-#if 0
-	movi	v0.8b, #1
-	movi	v9.16b, #2
-	movi	v31.8b, #3
-#endif
+	// This will reset ZA to all bits 0
+	smstop
+	smstart
 
 	ret
 endfunction

tools/testing/selftests/arm64/fp/zt-test.S

@@ -117,20 +117,16 @@ function check_zt
 	b	memcmp
 endfunction
 
-// Any SME register modified here can cause corruption in the main
-// thread -- but *only* the locations modified here.
+// Modify the live SME register state, signal return will undo our changes
 function irritator_handler
 	// Increment the irritation signal count (x23):
 	ldr	x0, [x2, #ucontext_regs + 8 * 23]
 	add	x0, x0, #1
 	str	x0, [x2, #ucontext_regs + 8 * 23]
 
-	// Corrupt some random ZT data
-#if 0
-	movi	v0.8b, #1
-	movi	v9.16b, #2
-	movi	v31.8b, #3
-#endif
+	// This will reset ZT to all bits 0
+	smstop
+	smstart
 
 	ret
 endfunction