x86/asm: Tidy up TSS limit code

In an earlier version of the patch ("x86/kvm/vmx: Defer TR reload
after VM exit") that introduced TSS limit validity tracking, I
confused which helper was which.  On reflection, the names I chose
sucked.  Rename the helpers to make it more obvious what's going on
and add some comments.

While I'm at it, clear __tss_limit_invalid when force-reloading as
well as when contitionally reloading, since any TR reload fixes the
limit.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/include/asm/desc.h

@@ -205,6 +205,8 @@ static inline void native_load_tr_desc(void)
 	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
 }
 
+DECLARE_PER_CPU(bool, __tss_limit_invalid);
+
 static inline void force_reload_TR(void)
 {
 	struct desc_struct *d = get_cpu_gdt_table(smp_processor_id());
@@ -220,18 +222,20 @@ static inline void force_reload_TR(void)
 	write_gdt_entry(d, GDT_ENTRY_TSS, &tss, DESC_TSS);
 
 	load_TR_desc();
+	this_cpu_write(__tss_limit_invalid, false);
 }
 
-DECLARE_PER_CPU(bool, need_tr_refresh);
-
-static inline void refresh_TR(void)
+/*
+ * Call this if you need the TSS limit to be correct, which should be the case
+ * if and only if you have TIF_IO_BITMAP set or you're switching to a task
+ * with TIF_IO_BITMAP set.
+ */
+static inline void refresh_tss_limit(void)
 {
 	DEBUG_LOCKS_WARN_ON(preemptible());
 
-	if (unlikely(this_cpu_read(need_tr_refresh))) {
+	if (unlikely(this_cpu_read(__tss_limit_invalid)))
 		force_reload_TR();
-		this_cpu_write(need_tr_refresh, false);
-	}
 }
 
 /*
@@ -250,7 +254,7 @@ static inline void invalidate_tss_limit(void)
 	if (unlikely(test_thread_flag(TIF_IO_BITMAP)))
 		force_reload_TR();
 	else
-		this_cpu_write(need_tr_refresh, true);
+		this_cpu_write(__tss_limit_invalid, true);
 }
 
 static inline void native_load_gdt(const struct desc_ptr *dtr)

arch/x86/kernel/ioport.c

@@ -47,8 +47,14 @@ asmlinkage long sys_ioperm(unsigned long from, unsigned long num, int turn_on)
 		t->io_bitmap_ptr = bitmap;
 		set_thread_flag(TIF_IO_BITMAP);
 
+		/*
+		 * Now that we have an IO bitmap, we need our TSS limit to be
+		 * correct.  It's fine if we are preempted after doing this:
+		 * with TIF_IO_BITMAP set, context switches will keep our TSS
+		 * limit correct.
+		 */
 		preempt_disable();
-		refresh_TR();
+		refresh_tss_limit();
 		preempt_enable();
 	}
 

arch/x86/kernel/process.c

@@ -65,8 +65,8 @@ __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
 };
 EXPORT_PER_CPU_SYMBOL(cpu_tss);
 
-DEFINE_PER_CPU(bool, need_tr_refresh);
-EXPORT_PER_CPU_SYMBOL_GPL(need_tr_refresh);
+DEFINE_PER_CPU(bool, __tss_limit_invalid);
+EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);
 
 /*
  * this gets called so that we can store lazy state into memory and copy the
@@ -218,7 +218,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 		 * Make sure that the TSS limit is correct for the CPU
 		 * to notice the IO bitmap.
 		 */
-		refresh_TR();
+		refresh_tss_limit();
 	} else if (test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) {
 		/*
 		 * Clear any possible leftover bits: