Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code

["H. Peter Anvin" <hpa@zytor.com>]

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Come to think about it, I think it would be better if we were to 
incorporate the X86_BUG_NULL_SEG null segment load in into the main 
native_load_gs_index() code while we are swapgs'd anyway. This 
simplifies the code further, and should avoid calling 
native_load_gs_index() twice if this bug is enabled.

(This is the traps.c code only; removing the now unnecessary instances 
is left as an exercise for the reader...)

	-hpa

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diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a58800973aed..5de423a24777 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -671,6 +671,48 @@ DEFINE_IDTENTRY_RAW(exc_int3)
 }
 
 #ifdef CONFIG_X86_64
+
+/*
+ * Reload the %gs selector for user space (setting GS_BASE) with
+ * exception handling: if the selector is invalid, set %gs to NULL.
+ *
+ * selector: new selector
+ *
+ * This function is noinstr as it must not be instrumented.
+ */
+noinstr void native_load_gs_index(unsigned int selector)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+
+again:
+	native_swapgs();
+
+	if (static_cpu_has_bug(X86_BUG_NULL_SEG)) {
+		if (likely(selector <= 3))
+			asm volatile("mov %[seg],%%gs"
+				     : : [seg] "rm" (__USER_DS));
+	}
+
+	asm_volatile_goto("1: mov %[seg],%%gs\n"
+			  _ASM_EXTABLE(1b, %l[bad_seg])
+			  : : [seg] "rm" (selector)
+			  : : bad_seg);
+
+	alternative("", "mfence", X86_BUG_SWAPGS_FENCE);
+	native_swapgs();
+	local_irq_restore(flags);
+	return;
+
+bad_seg:
+	/*
+	 * Invalid selector, set %gs to null (0).
+	 */
+	selector = 0;
+	goto again;
+}
+
 /*
  * Help handler running on a per-cpu (IST or entry trampoline) stack
  * to switch to the normal thread stack if the interrupted code was in