[PATCH v2 1/9] arm64: assembler: add cond_yield macro

[Ard Biesheuvel]

Add a macro cond_yield that branches to a specified label when called if
the TIF_NEED_RESCHED flag is set and decreasing the preempt count would
make the task preemptible again, resulting in a schedule to occur. This
can be used by kernel mode SIMD code that keeps a lot of state in SIMD
registers, which would make chunking the input in order to perform the
cond_resched() check from C code disproportionately costly.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/assembler.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index bf125c591116..27b1ea721c2d 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -745,6 +745,22 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
 .Lyield_out_\@ :
 	.endm
 
+	/*
+	 * Check whether preempt-disabled code should yield as soon as it
+	 * is able. This is the case if re-enabling preemption a single
+	 * time results in a preempt count of zero, and the TIF_NEED_RESCHED
+	 * flag is set. (Note that the latter is stored negated in the
+	 * top word of the thread_info::preempt_count field)
+	 */
+	.macro		cond_yield, lbl:req, tmp:req
+#ifdef CONFIG_PREEMPTION
+	get_current_task \tmp
+	ldr		\tmp, [\tmp, #TSK_TI_PREEMPT]
+	sub		\tmp, \tmp, #PREEMPT_DISABLE_OFFSET
+	cbz		\tmp, \lbl
+#endif
+	.endm
+
 /*
  * This macro emits a program property note section identifying
  * architecture features which require special handling, mainly for
-- 
2.30.0