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>

3 files changed, 21 insertions, 11 deletions

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index cb8f9149f6c8..1548ca92ad3f 100644
--- a/arch/x86/include/asm/desc.h
+++ b/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)
diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index b01bc8517450..875d3d25dd6a 100644
--- a/arch/x86/kernel/ioport.c
+++ b/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();
         }
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 7780efa635b9..0b302591b51f 100644
--- a/arch/x86/kernel/process.c
+++ b/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: