[PATCH v4 2/3] riscv: vector: allow non-preemptible kernel-mode vector with IRQs off

[Andy Chiu]

Similar to commit 7137a203b251 ("arm64/fpsimd: Permit kernel mode NEON
with IRQs off"), we are upgrading get/put_cpu_vector_context such that
kvm_arch_vcpu_load/put can be safely called under both irq off and
regular process context.

Also, export both symbols so the kvm module can call into it.

Signed-off-by: Andy Chiu <tchiu at tenstorrent.com>
---
Changelog v4:
 - Include the linux/kvm_types.h to prevent build fail (patchwork ci)
 - drop preempt_v_started check in may_use_simd()
Changelog v3:
 - Export {get,put}_cpu_vector_context() only to kvm modules (Sebastian)
Changelog v2:
 - new patch since v2
---
 arch/riscv/include/asm/simd.h          | 16 +++-------------
 arch/riscv/kernel/kernel_mode_vector.c | 19 +++++++++++++------
 2 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/arch/riscv/include/asm/simd.h b/arch/riscv/include/asm/simd.h
index adb50f3ec205..f176a8072c62 100644
--- a/arch/riscv/include/asm/simd.h
+++ b/arch/riscv/include/asm/simd.h
@@ -36,20 +36,10 @@ static __must_check inline bool may_use_simd(void)
 	/*
 	 * Nesting is achieved in preempt_v by spreading the control for
 	 * preemptible and non-preemptible kernel-mode Vector into two fields.
-	 * Always try to match with preempt_v if kernel V-context exists. Then,
-	 * fallback to check non preempt_v if nesting happens, or if the config
-	 * is not set.
+	 * Only non-preempt_v can nest on top of preempt_v, if non-preempt_v is
+	 * unavailable, then preempt_v is not allowed.
 	 */
-	if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE) && current->thread.kernel_vstate.datap) {
-		if (!riscv_preempt_v_started(current))
-			return true;
-	}
-	/*
-	 * Non-preemptible kernel-mode Vector temporarily disables bh. So we
-	 * must not return true on irq_disabled(). Otherwise we would fail the
-	 * lockdep check calling local_bh_enable()
-	 */
-	return !irqs_disabled() && !(riscv_v_flags() & RISCV_KERNEL_MODE_V);
+	return !(riscv_v_flags() & RISCV_KERNEL_MODE_V);
 }
 
 #else /* ! CONFIG_RISCV_ISA_V */
diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
index 307ac369c3d4..965c8edbe984 100644
--- a/arch/riscv/kernel/kernel_mode_vector.c
+++ b/arch/riscv/kernel/kernel_mode_vector.c
@@ -10,6 +10,7 @@
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/types.h>
+#include <linux/kvm_types.h>
 
 #include <asm/vector.h>
 #include <asm/switch_to.h>
@@ -55,13 +56,16 @@ void get_cpu_vector_context(void)
 	 * disable softirqs so it is impossible for softirqs to nest
 	 * get_cpu_vector_context() when kernel is actively using Vector.
 	 */
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_bh_disable();
-	else
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		if (!irqs_disabled())
+			local_bh_disable();
+	} else {
 		preempt_disable();
+	}
 
 	riscv_v_start(RISCV_KERNEL_MODE_V);
 }
+EXPORT_SYMBOL_FOR_KVM(get_cpu_vector_context);
 
 /*
  * Release the CPU vector context.
@@ -74,11 +78,14 @@ void put_cpu_vector_context(void)
 {
 	riscv_v_stop(RISCV_KERNEL_MODE_V);
 
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_bh_enable();
-	else
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		if (!irqs_disabled())
+			local_bh_enable();
+	} else {
 		preempt_enable();
+	}
 }
+EXPORT_SYMBOL_FOR_KVM(put_cpu_vector_context);
 
 #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
 static __always_inline u32 *riscv_v_flags_ptr(void)
-- 
2.43.0