[RFC PATCH 4/5] arm64: fpsimd: run kernel mode NEON with softirqs disabled

[Ard Biesheuvel <ardb@xxxxxxxxxx>]

Kernel mode NEON can be used in task or softirq context, but only in
a non-nesting manner, i.e., softirq context is only permitted if the
interrupt was not taken at a point where the kernel was using the NEON
in task context.

This means all users of kernel mode NEON have to be aware of this
limitation, and either need to provide scalar fallbacks that may be much
slower (up to 20x for AES instructions) and potentially less safe, or
use an asynchronous interface that defers processing to a later time
when the NEON is guaranteed to be available.

Given that grabbing and releasing the NEON is cheap, we can relax this
restriction, by increasing the granularity of kernel mode NEON code, and
always disabling softirq processing while the NEON is being used in task
context.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/arm64/include/asm/assembler.h | 19 +++++++++++++------
 arch/arm64/kernel/asm-offsets.c    |  2 ++
 arch/arm64/kernel/fpsimd.c         |  4 ++--
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index ddbe6bf00e33..74ce46ed55ac 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -15,6 +15,7 @@
 #include <asm-generic/export.h>
 
 #include <asm/asm-offsets.h>
+#include <asm/alternative.h>
 #include <asm/cpufeature.h>
 #include <asm/cputype.h>
 #include <asm/debug-monitors.h>
@@ -717,17 +718,23 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
 	.endm
 
 	.macro		if_will_cond_yield_neon
-#ifdef CONFIG_PREEMPTION
 	get_current_task	x0
 	ldr		x0, [x0, #TSK_TI_PREEMPT]
-	sub		x0, x0, #PREEMPT_DISABLE_OFFSET
-	cbz		x0, .Lyield_\@
+#ifdef CONFIG_PREEMPTION
+	cmp		x0, #PREEMPT_DISABLE_OFFSET
+	beq		.Lyield_\@	// yield on need_resched in task context
+#endif
+	/* never yield while serving a softirq */
+	tbnz		x0, #SOFTIRQ_SHIFT, .Lnoyield_\@
+
+	adr_l		x0, irq_stat + IRQ_CPUSTAT_SOFTIRQ_PENDING
+	this_cpu_offset	x1
+	ldr		w0, [x0, x1]
+	cbnz		w0, .Lyield_\@	// yield on pending softirq in task context
+.Lnoyield_\@:
 	/* fall through to endif_yield_neon */
 	.subsection	1
 .Lyield_\@ :
-#else
-	.section	".discard.cond_yield_neon", "ax"
-#endif
 	.endm
 
 	.macro		do_cond_yield_neon
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 7d32fc959b1a..34ef70877de4 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -93,6 +93,8 @@ int main(void)
   DEFINE(DMA_FROM_DEVICE,	DMA_FROM_DEVICE);
   BLANK();
   DEFINE(PREEMPT_DISABLE_OFFSET, PREEMPT_DISABLE_OFFSET);
+  DEFINE(SOFTIRQ_SHIFT, SOFTIRQ_SHIFT);
+  DEFINE(IRQ_CPUSTAT_SOFTIRQ_PENDING, offsetof(irq_cpustat_t, __softirq_pending));
   BLANK();
   DEFINE(CPU_BOOT_STACK,	offsetof(struct secondary_data, stack));
   DEFINE(CPU_BOOT_TASK,		offsetof(struct secondary_data, task));
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 062b21f30f94..823e3a8a8871 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -180,7 +180,7 @@ static void __get_cpu_fpsimd_context(void)
  */
 static void get_cpu_fpsimd_context(void)
 {
-	preempt_disable();
+	local_bh_disable();
 	__get_cpu_fpsimd_context();
 }
 
@@ -201,7 +201,7 @@ static void __put_cpu_fpsimd_context(void)
 static void put_cpu_fpsimd_context(void)
 {
 	__put_cpu_fpsimd_context();
-	preempt_enable();
+	local_bh_enable();
 }
 
 static bool have_cpu_fpsimd_context(void)
-- 
2.17.1