arm64: assembler: add macros to conditionally yield the NEON under PREEMPT

Add support macros to conditionally yield the NEON (and thus the CPU)
that may be called from the assembler code.

In some cases, yielding the NEON involves saving and restoring a non
trivial amount of context (especially in the CRC folding algorithms),
and so the macro is split into three, and the code in between is only
executed when the yield path is taken, allowing the context to be preserved.
The third macro takes an optional label argument that marks the resume
path after a yield has been performed.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index fe2ff3efe1f077970c1276ab4ca4619439334179..0bcc98dbba565f1727995b5ec16ff3e978720174 100644 (file)
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -628,4 +628,77 @@ USER(\label, ic    ivau, \tmp2)                    // invalidate I line PoU
        .endif
        .endm
 
+/*
+ * Check whether to yield to another runnable task from kernel mode NEON code
+ * (which runs with preemption disabled).
+ *
+ * if_will_cond_yield_neon
+ *        // pre-yield patchup code
+ * do_cond_yield_neon
+ *        // post-yield patchup code
+ * endif_yield_neon    <label>
+ *
+ * where <label> is optional, and marks the point where execution will resume
+ * after a yield has been performed. If omitted, execution resumes right after
+ * the endif_yield_neon invocation. Note that the entire sequence, including
+ * the provided patchup code, will be omitted from the image if CONFIG_PREEMPT
+ * is not defined.
+ *
+ * As a convenience, in the case where no patchup code is required, the above
+ * sequence may be abbreviated to
+ *
+ * cond_yield_neon <label>
+ *
+ * Note that the patchup code does not support assembler directives that change
+ * the output section, any use of such directives is undefined.
+ *
+ * The yield itself consists of the following:
+ * - Check whether the preempt count is exactly 1, in which case disabling
+ *   preemption once will make the task preemptible. If this is not the case,
+ *   yielding is pointless.
+ * - Check whether TIF_NEED_RESCHED is set, and if so, disable and re-enable
+ *   kernel mode NEON (which will trigger a reschedule), and branch to the
+ *   yield fixup code.
+ *
+ * This macro sequence may clobber all CPU state that is not guaranteed by the
+ * AAPCS to be preserved across an ordinary function call.
+ */
+
+       .macro          cond_yield_neon, lbl
+       if_will_cond_yield_neon
+       do_cond_yield_neon
+       endif_yield_neon        \lbl
+       .endm
+
+       .macro          if_will_cond_yield_neon
+#ifdef CONFIG_PREEMPT
+       get_thread_info x0
+       ldr             w1, [x0, #TSK_TI_PREEMPT]
+       ldr             x0, [x0, #TSK_TI_FLAGS]
+       cmp             w1, #PREEMPT_DISABLE_OFFSET
+       csel            x0, x0, xzr, eq
+       tbnz            x0, #TIF_NEED_RESCHED, .Lyield_\@       // needs rescheduling?
+       /* fall through to endif_yield_neon */
+       .subsection     1
+.Lyield_\@ :
+#else
+       .section        ".discard.cond_yield_neon", "ax"
+#endif
+       .endm
+
+       .macro          do_cond_yield_neon
+       bl              kernel_neon_end
+       bl              kernel_neon_begin
+       .endm
+
+       .macro          endif_yield_neon, lbl
+       .ifnb           \lbl
+       b               \lbl
+       .else
+       b               .Lyield_out_\@
+       .endif
+       .previous
+.Lyield_out_\@ :
+       .endm
+
 #endif /* __ASM_ASSEMBLER_H */

diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 78e1b0a70aaf37a4bb14cc24de36e1801fedc56f..5bdda651bd0507f5c2fa96b85ae8309ff2692a0b 100644 (file)
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -23,6 +23,7 @@
 #include <linux/mm.h>
 #include <linux/dma-mapping.h>
 #include <linux/kvm_host.h>
+#include <linux/preempt.h>
 #include <linux/suspend.h>
 #include <asm/cpufeature.h>
 #include <asm/fixmap.h>
@@ -93,6 +94,8 @@ int main(void)
   DEFINE(DMA_TO_DEVICE,                DMA_TO_DEVICE);
   DEFINE(DMA_FROM_DEVICE,      DMA_FROM_DEVICE);
   BLANK();
+  DEFINE(PREEMPT_DISABLE_OFFSET, PREEMPT_DISABLE_OFFSET);
+  BLANK();
   DEFINE(CLOCK_REALTIME,       CLOCK_REALTIME);
   DEFINE(CLOCK_MONOTONIC,      CLOCK_MONOTONIC);
   DEFINE(CLOCK_MONOTONIC_RAW,  CLOCK_MONOTONIC_RAW);