1 files changed, 21 insertions, 13 deletions

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 91d6341f281f..399f43aa78d5 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -43,8 +43,11 @@ struct fpu_state_config fpu_user_cfg __ro_after_init;
  */
 struct fpstate init_fpstate __ro_after_init;
 
-/* Track in-kernel FPU usage */
-static DEFINE_PER_CPU(bool, in_kernel_fpu);
+/*
+ * Track FPU initialization and kernel-mode usage. 'true' means the FPU is
+ * initialized and is not currently being used by the kernel:
+ */
+DEFINE_PER_CPU(bool, kernel_fpu_allowed);
 
 /*
  * Track which context is using the FPU on the CPU:
@@ -61,15 +64,18 @@ bool irq_fpu_usable(void)
 		return false;
 
 	/*
-	 * In kernel FPU usage already active?  This detects any explicitly
-	 * nested usage in task or softirq context, which is unsupported.  It
-	 * also detects attempted usage in a hardirq that has interrupted a
-	 * kernel-mode FPU section.
+	 * Return false in the following cases:
+	 *
+	 * - FPU is not yet initialized. This can happen only when the call is
+	 *   coming from CPU onlining, for example for microcode checksumming.
+	 * - The kernel is already using the FPU, either because of explicit
+	 *   nesting (which should never be done), or because of implicit
+	 *   nesting when a hardirq interrupted a kernel-mode FPU section.
+	 *
+	 * The single boolean check below handles both cases:
 	 */
-	if (this_cpu_read(in_kernel_fpu)) {
-		WARN_ON_FPU(!in_hardirq());
+	if (!this_cpu_read(kernel_fpu_allowed))
 		return false;
-	}
 
 	/*
 	 * When not in NMI or hard interrupt context, FPU can be used in:
@@ -431,9 +437,10 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
 		fpregs_lock();
 
 	WARN_ON_FPU(!irq_fpu_usable());
-	WARN_ON_FPU(this_cpu_read(in_kernel_fpu));
 
-	this_cpu_write(in_kernel_fpu, true);
+	/* Toggle kernel_fpu_allowed to false: */
+	WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, false);
 
 	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
 	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
@@ -453,9 +460,10 @@ EXPORT_SYMBOL_GPL(kernel_fpu_begin_mask);
 
 void kernel_fpu_end(void)
 {
-	WARN_ON_FPU(!this_cpu_read(in_kernel_fpu));
+	/* Toggle kernel_fpu_allowed back to true: */
+	WARN_ON_FPU(this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, true);
 
-	this_cpu_write(in_kernel_fpu, false);
 	if (!irqs_disabled())
 		fpregs_unlock();
 }